This document provides an introduction and overview of embedded microcontrollers and the process of learning to program them. It discusses how microcontrollers are now found in many everyday appliances and products. It describes how the cost of getting started with microcontroller development has decreased significantly in recent years, with microcontrollers now having electrically erasable program memory and low-cost development kits available. It promotes C as a programming language that is better than assembly for microcontroller programming, being more readable and maintainable. The document introduces a book that guides readers through developing microcontroller applications in C from basic to complex using the PIC16F877A microcontroller, MPLAB IDE, and Proteus simulation software.

2. Foreword
Embedded microcontrollers are everywhere today. In the average household you will
find them far beyond the obvious places like cell phones, calculators, and MP3 players.
Hardly any new appliance arrives in the home without at least one controller and, most
likely, there will be several—one microcontroller for the user interface (buttons and
display), another to control the motor, and perhaps even an overall system manager. This
applies whether the appliance in question is a washing machine, garage door opener,
curling iron, or toothbrush. If the product uses a rechargeable battery, modern high
density battery chemistries require intelligent chargers.
A decade ago, there were significant barriers to learning how to use microcontrollers.
The cheapest programmer was about a hundred dollars and application development
required both erasable windowed parts—which cost about ten times the price of the
one time programmable (OTP) version—and a UV Eraser to erase the windowed part.
Debugging tools were the realm of professionals alone. Now most microcontrollers use
Flash-based program memory that is electrically erasable. This means the device can be
reprogrammed in the circuit—no UV eraser required and no special packages needed for
development. The total cost to get started today is about twenty-five dollars which buys
a PICkit™ 2 Starter Kit, providing programming and debugging for many Microchip
Technology Inc. MCUs. Microchip Technology has always offered a free Integrated
Development Environment (IDE) including an assembler and a simulator. It has never
been less expensive to get started with embedded microcontrollers than it is today.
While MPLAB®
includes the assembler for free, assembly code is more cumbersome
to write, in the first place, and also more difficult to maintain. Developing code using
C frees the programmer from the details of multi-byte math and paging and generally
improves code readability and maintainability. CCS and Hi-Tech both offer free “student”
versions of the compiler to get started and even the full versions are relatively inexpensive
once the savings in development time has been taken into account.
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While the C language eliminates the need to learn the PIC16 assembly language and frees
the user from managing all the details, it is still necessary to understand the architecture.
Clocking options, peripherals sets, and pin multiplexing issues still need to be solved.
Martin’s book guides readers, step-by-step, on the journey from “this is a micro-
controller” to “here’s how to complete an application.” Exercises use the fully featured
PIC16F877A, covering the architecture and device configuration. This is a good starting
point because other PIC16s are similar in architecture but differ in terms of IO lines,
memory, or peripheral sets. An application developed on the PIC16F877A can easily be
transferred to a smaller and cheaper midrange PICmicro. The book also introduces the
peripherals and shows how they can simplify the firmware by letting the hardware do the
work.
MPLAB®
, Microchip’s Integrated Development Environment, is also covered. MPLAB
includes an editor and a simulator and interfaces with many compilers, including the
CCS compiler used in this book. Finally, the book includes the Proteus®
simulator which
allows complete system simulation, saving time and money on prototype PCBs.
Dan Butler
Principal Applications Engineer
Microchip Technology Inc.
xii Foreword
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Preface
This book is the third in a series, including
● PIC Microcontrollers: An Introduction to Microelectronic Systems.
● Interfacing PIC Microcontrollers: Embedded Design by Interactive Simulation.
● Programming 8-bit PIC Microcontrollers in C: With Interactive Hardware
Simulation.
It completes a set that introduces embedded application design using the Microchip
PIC®
range, from Microchip Technology Inc. of Arizona. This is the most popular
microcontroller for education and training, which is also rapidly gaining ground in the
industrial and commercial sectors. Interfacing PIC Microcontrollers and Programming
PIC Microcontrollers present sample applications using the leading design and simulation
software for microcontroller based circuits, Proteus VSM®
from Labcenter Electronics.
Demo application files can be downloaded from the author’s support Web site (see
later for details) and run on-screen so that the operation of each program can be studied
in detail.
The purpose of this book is to
● Introduce C programming specifically for microcontrollers in easy steps.
● Demonstrate the use of the Microchip MPLAB IDE for C projects.
● Provide a beginners’ guide to the CCS PCM C compiler for 16 series PICs.
● Explain how to use Proteus VSM to test C applications in simulated hardware.
● Describe applications for the Microchip PICDEM mechatronics board.
● Outline the principles of embedded system design and project development.
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C is becoming the language of choice for embedded systems, as memory capacity
increases in microcontrollers. Microchip supplies the 18 and 24 series chips specifically
designed for C programming. However, C can be used in the less complex 16 series PIC,
as long as the applications are relatively simple and therefore do not exceed the more
limited memory capacity.
The PIC 16F877A microcontroller is used as the reference device in this book, as it
contains a full range of peripherals and a reasonable memory capacity. It was also used
in the previous work on interfacing, so there is continuity if the book series is taken as a
complete course in PIC application development.
Microcontrollers are traditionally programmed in assembly language, each type having
its own syntax, which translates directly into machine code. Some students, teachers, and
hobbyists may wish to skip a detailed study of assembler coding and go straight to C,
which is generally simpler and more powerful. It is therefore timely to produce a text that
does not assume detailed knowledge of assembler and introduces C as gently as possible.
Although several C programming books for microcontrollers are on the market, many
are too advanced for the C beginner and distract the learner with undesirable detail in the
early stages.
This text introduces embedded programming techniques using the simplest possible
programs, with on-screen, fully interactive circuit simulation to demonstrate a range of
basic techniques, which can then be applied to your own projects. The emphasis is on
simple working programs for each topic, with hardware block diagrams to clarify system
operation, full circuit schematics, simulation screenshots, and source code listings, as
well as working downloads of all examples. Students in college courses and design
engineers can document their projects to a high standard using these techniques. Each
part concludes with a complete set of self-assessment questions and assignments designed
to complete the learning package.
An additional feature of this book is the use of Proteus VSM (virtual system modeling).
The schematic capture component, ISIS, allows a circuit diagram to be created using an
extensive library of active components. The program is attached to the microcontroller,
and the animated schematic allows the application to be comprehensively debugged
before downloading to hardware. This not only saves time for the professional engineer
but provides an excellent learning tool for the student or hobbyist.
xiv Preface
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Links, Resources, and Acknowledgments
Microchip Technology Inc. (www.microchip.com)
Microchip Technology Inc. is a manufacturer of PIC®
microcontrollers and associated
products. I gratefully acknowledge the support and assistance of Microchip Inc. in
the development of this book and the use of the company trademarks and intellectual
property. Special thanks are due to John Roberts of Microchip UK for his assistance
and advice. The company Web site contains details of all Microchip hardware, software,
and development systems. MPLAB IDE (integrated development system) must be
downloaded and installed to develop new applications using the tools described in this
book. The data sheet for the PIC 16F877A microcontroller should also be downloaded as
a reference source.
PIC, PICmicro, MPLAB, MPASM, PICkit, dsPIC, and PICDEM are trademarks of
Microchip Technology Inc.
Labcenter Electronics (www.labcenter.co.uk)
Labcenter Electronics is the developer of Proteus VSM (virtual system modeling), the
most advanced cosimulation system for embedded applications. I gratefully acknowledge
the assistance of the Labcenter team, especially John Jameson, in the development of
this series of books. A student/evaluation version of the simulation software may be
downloaded from www.proteuslite.com. A special offer for ISIS Lite, ProSPICE Lite,
and the 16F877A simulator model can be found at www.proteuslite.com/register/
ipmbundle.htm.
Proteus VSM, ISIS, and ARES are trademarks of Labcenter Electronics Ltd.
Custom Computer Services Inc. (www.ccsinfo.com)
Custom Computer Services Inc. specializes in compilers for PIC microcontrollers. The
main range comprises PCB compiler for 12-bit PICs, PCM for 16-bit, and PCH for
the 18 series chips. The support provided by James Merriman at CCS Inc. is gratefully
acknowledged. The manual for the CCS compiler should be downloaded from the
company Web site (Version 4 was used for this book). A 30-day trial version, which will
compile code for the 16F877A, is available at the time of writing.
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The Author’s Web Site (www.picmicros.org.uk)
This book is supported by a dedicated Web site, www.picmicros.org.uk. All the
application examples in the book may be downloaded free of charge and tested using
an evaluation version of Proteus VSM. The design files are locked so that the hardware
configuration cannot be changed without purchasing a suitable VSM license. Similarly,
the attached program cannot be modified and recompiled without a suitable compiler
license, available from the CCS Web site. Special manufacturer’s offers are available via
links at my site. This site is hosted by www.larrytech.com and special thanks are due to
Gabe Hudson of Larrytech®
Internet Services for friendly maintenance and support.
I can be contacted at the e-mail address martin@picmicros.org.uk with any queries or
comments related to the PIC book series.
Finally, thanks to Julia for doing the boring domestic stuff so I can do the interesting
technical stuff.
About the Author
Martin P. Bates is the author of PIC Microcontrollers, Second Edition. He is currently
lecturing on electronics and electrical engineering at Hastings College, UK. His interests
include microcontroller applications and embedded system design.
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Introduction
The book is organized in five parts. Part 1 includes an overview of the PIC microcontroller
internal architecture, describing the features of the 16F877A specifically. This chip is
often used as representative of the 16 series MCUs because it has a full range of
peripheral interfaces. All 16 series chips have a common program execution core, with
variation mainly in the size of program and data memory. During programming, certain
operational features are configurable: type of clock circuit, watchdog timer enable, reset
mechanisms, and so on. Internal features include the file register system, which contains
the control registers and RAM block, and a nonvolatile EEPROM block. The parallel
ports provide the default I/O for the MCU, but most pins have more than one function.
Eight analog inputs and serial interfaces (UART, SPI, and I2
C) are brought out to specific
pins. The hardware features of all these are outlined, so that I/O programming can be
more readily understood later on. The application development process is described,
using only MPLAB IDE in this initial phase. A sample C program is edited, compiled,
downloaded, and tested to demonstrate the basic process and the generated file set
analyzed. The debugging features of MPLAB are also outlined: run, single step,
breakpoints, watch windows, and so on. Disassembly of the object code allows the
intermediate assembly language version of the C source program to be analyzed.
Part 2 introduces C programming, using the simplest possible programs. Input and output
are dealt with immediately, since this is the key feature of embedded programs. Variables,
conditional blocks (IF), looping (WHILE,FOR) are quickly introduced, with a complete
example program. Variables and sequence control are considered in a little more detail
and functions introduced. This leads on to library functions for operating timers and
ports. The keypad and alphanumeric LCD are used in a simple calculator program. More
data types (long integers, floating point numbers, arrays, etc.) follow as well as assembler
directives and the purpose of the header file. Finally, insertion of assembler into C
programs is outlined.
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Part 3 focuses on programming input and output operations using the CCS C library
functions. These simplify the programming process, with a small set of functions usually
providing all the initialization and operating sequences required. Example programs
for analog input and the use of interrupts and timers are developed and the serial port
functions demonstrated in sample applications. The advantages of each type of serial bus
are compared, and examples showing the connection of external serial EEPROM for data
storage and a digital to analog converter output are provided. These applications can be
tested in VSM, but this is not essential; use of VSM is optional throughout the book.
Part 4 focuses specifically on the PICDEM mechatronics board from Microchip. This has
been selected as the main demonstration application, as it is relatively inexpensive and
contains a range of features that allow the features of a typical mechatronics system to
be examined: input sensors (temperature, light, and position) and output actuators (DC
and stepper motor). These are tested individually then the requirements of a temperature
controller outlined. Operation of the 3.5-digit seven-segment LCD is explained in detail,
as this is not covered elsewhere. A simulation version of the board is provided to aid
further application design and implementation.
Part 5 outlines some principles of software and hardware design and provides some
further examples. A simple temperature controller provides an alternative design to that
based on the mechatronics board, and a data logger design is based on another standard
hardware system, which can be adapted to a range of applications—the BASE board.
Again, a full-simulation version is provided for testing and further development work.
This is followed by a section on operating systems, which compares three program
design options: a polling loop, interrupt driven systems, and real-time operating systems.
Consideration of criteria for the final selection of the MCU for a given application and
some general design points follow.
Three appendices (A, B, and C) cover hardware design using ISIS schematic capture,
software design using CCS C, and system testing using Proteus VSM. These topics are
separated from the main body of the book as they are related more to specific products.
Taken together, MPLAB, CCS C, and Proteus VSM constitute a complete learning/design
package, but using them effectively requires careful study of product-specific tutorials.
VSM, in particular, has comprehensive, well-designed help files; and it is therefore
unnecessary to duplicate that material here. Furthermore, as with all good design tools,
VSM evolves very quickly, so a detailed tutorial quickly becomes outdated.
Appendix D compares alternative compilers, and application development areas are
identified that would suit each one. Appendix E provides a summary of CCS C syntax
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10. Introduction xix
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requirements, and Appendix F contains a list of the CCS C library functions provided
with the compiler, organized in functional groups for ease of reference. These are
intended to provide a convenient reference source when developing CCS C programs, in
addition to the full CCS compiler reference manual.
Each part of the book is designed to be as self-contained as possible, so that parts can be
skipped or studied in detail, depending on the reader’s previous knowledge and interests.
On the other hand, the entire book should provide a coherent narrative leading to a solid
grounding in C programming for embedded systems in general.
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PIC Microcontroller Systems
1.1 PIC16 Microcontrollers
● MCU features
● Program execution
● RAM file registers
● Other PIC chips
The microcontroller unit (MCU) is now big, or rather small, in electronics. It is one of the
most significant developments in the continuing miniaturization of electronic hardware.
Now, even trivial products, such as a musical birthday card or electronic price tag, can
include an MCU. They are an important factor in the digitization of analog systems, such
as sound systems or television. In addition, they provide an essential component of larger
systems, such as automobiles, robots, and industrial systems. There is no escape from
microcontrollers, so it is pretty useful to know how they work.
The computer or digital controller has three main elements: input and output devices,
which communicate with the outside world; a processor, to make calculations and handle
data operations; and memory, to store programs and data. Figure 1.1 shows these in a
little more detail. Unlike the conventional microprocessor system (such as a PC), which
has separate chips on a printed circuit board, the microcontroller contains all these
elements in one chip. The MCU is essentially a computer on a chip; however, it still
needs input and output devices, such as a keypad and display, to form a working system.
The microcontroller stores its program in ROM (read only memory). In the past, UV
(ultraviolet) erasable programmable ROM (EPROM) was used for prototyping or
P A R T 1
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small batch production, and one-time programmable ROM for longer product runs.
Programmable ROM chips are programmed in the final stages of manufacture, while
EPROM could be programmed by the user.
Flash ROM is now normally used for prototyping and low-volume production. This can
be programmed in circuit by the user after the circuit has been built. The prototyping
cycle is faster, and software variations are easier to accommodate. We are all now familiar
with flash ROM as used in USB memory sticks, digital camera memory, and so on, with
Gb (109
byte) capacities commonplace.
The range of microcontrollers available is expanding rapidly. The first to be widely used,
the Intel 8051, was developed alongside the early Intel PC processors, such as the 8086.
This device dominated the field for some time; others emerged only slowly, mainly
in the form of complex processors for applications such as engine management systems.
These devices were relatively expensive, so they were justified only in high-value
products. The potential of microcontrollers seems to have been realized only slowly.
The development of flash ROM helped open up the market, and Microchip was among
the first to take advantage. The cheap and reprogrammable PIC16F84 became the most
widely known, rapidly becoming the number one device for students and hobbyists. On
the back of this success, the Microchip product range rapidly developed and diversified.
The supporting development system, MPLAB, was distributed free, which helped the PIC
to dominate the low-end market.
Flash ROM is one of the technical developments that made learning about microsystems
easier and more interesting. Interactive circuit design software is another. The whole
design process is now much more transparent, so that working systems are more quickly
achievable by the beginner. Low-cost in-circuit debugging is another technique that
helps get the final hardware up and running quickly, with only a modest expenditure on
development tools.
User Input User Output
Input
Peripherals
Output
Peripherals
RAM
Read & Write
Memory
CPU
Central
Processing
Unit
ROM
Read Only
Memory
Program
Download
Figure 1.1: Elements of a Digital Controller
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MCU Features
The range of microcontrollers now available developed because the features of the MCU
used in any particular circuit must be as closely matched as possible to the actual needs of
the application. Some of the main features to consider are
● Number of inputs and outputs.
● Program memory size.
● Data RAM size.
● Nonvolatile data memory.
● Maximum clock speed.
● Range of interfaces.
● Development system support.
● Cost and availability.
The PIC16F877A is useful as a reference device because it has a minimal instruction
set but a full range of peripheral features. The general approach to microcontroller
application design followed here is to develop a design using a chip that has spare
capacity, then later select a related device that has the set of features most closely
matching the application requirements. If necessary, we can drop down to a lower range
(PIC10/12 series), or if it becomes clear that more power is needed, we can move up
to a higher specification chip (PIC18/24 series). This is possible as all devices have
the same core architecture and compatible instructions sets.
The most significant variation among PIC chips is the instruction size, which can be
12, 14, or 16 bits. The A suffix indicates that the chip has a maximum clock speed of
20MHz, the main upgrade from the original 16F877 device. These chips can otherwise be
regarded as identical, the suffix being optional for most purposes. The 16F877A pin-out
is seen in Figure 1.2 and the internal architecture in Figure 1.3. The latter is a somewhat
simplified version of the definitive block diagram in the data sheet.
Program Execution
The chip has 8k (8096 ⫻ 14 bits) of flash ROM program memory, which has to be
programmed via the serial programming pins PGM, PGC, and PGD. The fixed-length
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instructions contain both the operation code and operand (immediate data, register
address, or jump address). The mid-range PIC has a limited number of instructions (35)
and is therefore classified as a RISC (reduced instruction set computer) processor.
Looking at the internal architecture, we can identify the blocks involved in program
execution. The program memory ROM contains the machine code, in locations numbered
from 0000h to 1FFFh (8k). The program counter holds the address of the current
instruction and is incremented or modified after each step. On reset or power up, it is reset
to zero and the first instruction at address 0000 is loaded into the instruction register,
decoded, and executed. The program then proceeds in sequence, operating on the contents
of the file registers (000–1FFh), executing data movement instructions to transfer data
between ports and file registers or arithmetic and logic instructions to process it. The CPU
has one main working register (W), through which all the data must pass.
If a branch instruction (conditional jump) is decoded, a bit test is carried out; and if
the result is true, the destination address included in the instruction is loaded into the
program counter to force the jump. If the result is false, the execution sequence continues
unchanged. In assembly language, when CALL and RETURN are used to implement
RB7/PGD
MCLR/VPP
RA0/AN0 RB6/PGC
RB5
RB4
RB3/PGM
RB2
RB1
RB0/INT
VDD
VSS
RD7/PSP7
RD6/PSP6
RD5/PSP5
RD4/PSP4
RC7/RX/DT
RC6/TX/CK
RC5/SDO
RC4/SDI/SDA
RD3/PSP3
RD2/PSP2
RA1/AN1
RA2/AN2/VREF⫺/CVREF
RA3/AN3/VREF⫹
RA4/T0CKI/C1OUT
RA5/AN4/SS/C2OUT
RE0/RD/AN5
RE1/WR/AN6
RE2/CS/AN7
VDD
VSS
OSC1/CLKI
OSC2/CLKO
RC0/T1OSO/T1CKI
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RD0/PSP0
RD1/PSP1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
PIC16F874A/877A
Figure 1.2: 16F877 Pin-out (reproduced by permission of Microchip Inc.)
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subroutines, a similar process occurs. The stack is used to store return addresses, so
that the program can return automatically to the original program position. However,
this mechanism is not used by the CCS C compiler, as it limits the number of levels of
subroutine (or C functions) to eight, which is the depth of the stack. Instead, a simple
GOTO instruction is used for function calls and returns, with the return address computed
by the compiler.
Flash
ROM
Program
Memory
8192
⫻ 14 bits
Program Counter
(13 bits)
Address
Instructions
File Address
Program Address
Stack
13 bits
⫻ 8
Levels
RAM
File
Registers
368
⫻ 8 bits
Instruction Register
File Select
Register
MCU
control
lines
Working (W)
Register
Arithmetic & Logic
Unit
Status (Flag)
Register
Literal
Status
Op-
code Data Bus
(8 bits)
Instruction
Decode &
CPU control
EEPROM
256 bytes
Ports, Timers
ADC, Serial I/O
Clock Reset
Port A B C D E
Timing control
Figure 1.3: PIC16F877 MCU Block Diagram
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Table 1.1: PIC16F877 Simplified File Register Map
Bank 0 (000–07F) Bank 1 (080–0FF) Bank 2 (100–180) Bank 3 (180–1FF)
Address Register Address Register Address Register Address Register
000h Indirect 080h Indirect 100h Indirect 180h Indirect
001h Timer0 081h Option 101h Timer0 181h Option
002h
Prog.
count.
low
082h
Prog.
count.
low
102h
Prog.
count.
low
182h
Prog.
count.
low
003h Status reg 083h Status reg 103h Status reg 183h Status reg
004h File select 084h File select 104h File select 184h File select
005h
Port A
data
085h
Port A
direction
105h — 185h —
006h Port B
data
086h Port B
direction
106h
Port B
data
186h Port B
direction
007h
Port C
data
087h
Port C
direction
107h — 187h —
008h
Port D
data
088h
Port D
direction
108h — 188h —
009h
Port E
data
089h
Port E
direction
109h — 189h —
00Ah
Prog.
count.
high
08Ah
Prog.
count.
high
10Ah
Prog.
count.
high
18Ah
Prog.
count.
high
00Bh
Interrupt
control
08Bh
Interrupt
control
10Bh
Interrupt
control
18Bh
Interrupt
control
00Ch–
01Fh
20
peripheral
control
registers
08Ch–
09Fh
20
peripheral
control
registers
10Ch–
10Fh
4
peripheral
control
registers
18Ch–
18Fh
4
peripheral
control
registers
110h–
11Fh
16 general
purpose
registers
190h–
19Fh
16 general
purpose
registers
020h–
06Fh
80 general
purpose
registers
0A0h–
0EFh
80 general
purpose
registers
120h–
16Fh
80 general
purpose
registers
1A0h–
1EFh
80 general
purpose
registers
070h–
07Fh
16
common
access
GPRs
0F0h–
0FFh
Accesses
070h–
07Fh
170h–
17Fh
Accesses
070h–
07Fh
1F0h–
1FFh
Accesses
070h–
07Fh
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RAM File Registers
The main RAM block (Table 1.1) is a set of 368 8-bit file registers, including the special
function registers (SFRs), which have a dedicated function, and the general purpose
registers (GPRs). When variables are created in C, they are stored in the GPRs, starting at
address 0020h. The file registers are divided into four blocks, register banks 0 to 3. The
SFRs are located at the low addresses in each RAM bank.
Some registers are addressable across the bank boundaries; for example, the status
register can be accessed in all blocks at the corresponding address in each bank. Others
are addressable in only a specific page, for example, Port A data register. Some register
addresses are not physically implemented. Since some registers are accessible in multiple
banks, bank switching can be minimized by the compiler when assembling the machine
code, thus saving program code space and execution time. For full details of the file
register set, see the MCU data sheet.
The program counter uses two 8-bit registers to store a 13-bit program memory address.
Only the low byte at address 002h is directly addressable. The status register 003h
records results from ALU (arithmetic and logic unit) operations, such as zero and carry/
borrow. The indirect and file select registers are used for indexed addressing of the GPRs.
Timer0 is the timer/counter register available in all PIC MCUs, while Timer1 and Timer2
registers are in the peripheral block. The port registers are located in Bank 0 at addresses
05h (Port A) to 09h (Port E) with the data direction register for each at the corresponding
location in bank 1. We can see that a total of 80 ⫹ 16 ⫹ 80 ⫹ 96 ⫹ 96 ⫽ 368 GPRs are
available for use as data RAM. Note that the number of registers used for each C variable
depends on the variable type and can range from 1 to 32 bits (1–4 GPRs).
Other PIC Chips
In any embedded design, the features of the MCU need to be matched to the application
requirements. The manufacturer needs to make sure that, as applications become more
demanding, a more powerful device of a familiar type is available. We can see this
process at work where Microchip started out producing basic chips such as the 16C84,
then developed the product range to meet the growing market. PIC microcontrollers are
currently available in distinct groups, designated the 10, 12, 16, 18, and 24 series. Their
general characteristics are outlined in Table 1.2.
The original 16 series CMOS devices were designated as 16CXX. When flash memory
was introduced, they became 16FXXX. Currently, a limited number of devices are
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available in the low pin count (LPC) ranges (10/12 series), while the power ranges are
expanding rapidly. In addition are those listed in the 24HXXXX range, which runs at 40
MIPS, and the dsPIC (digital signal processor) high-specification range.
1.2 PIC16 MCU Configuration
● Clock oscillator types
● Watchdog, power-up, brown-out timers
● Low-voltage programming
● Code protection
● In-circuit debug mode
When programming the PIC microcontroller, certain operational modes must be set
prior to the main program download. These are controlled by individual bits in a special
Table 1.2: PIC Microcontroller Types
MCU Pins Data
Word
(bits)
Program
Memory
(bytes)
Typical
Instruction
Set
Speed
MIPS
Description
10FXXX ⫽ 6 8 ⱕ512 33 ⫻ 12 bits ⱕ2
Low pin count, small
form factor, cheap, no
EEPROM, no low-power,
assembler program
12FXXX ⫽ 8 8 ⱕ2 kB 12/14 bits ⱕ0.5
Low pin count, small form
factor, cheap, EEPROM,
10-bit ADC, some low
power, assembler
16FXXX ⱕ64 8 ⱕ14 kB 35 ⫻ 14 bits ⱕ5
Mid-range, UART, I2C,
SPI, many low power, C or
assembler program
18FXXXX ⱕ100 8 ⱕ128 kB 75 ⫻ 16 bits ⱕ16
High range, CAN, USB
J series 3V supply, C
program
24FXXXX ⱕ100 16 ⱕ128 kB 76 ⫻ 24 bits ⫽ 16
Power range, 3V supply,
no EEPROM, data RAM
ⱕ8 kB, C program
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configuration register separated from the main memory block. The main options are as
follows.
Clock Options
The ‘877 chip has two main clock modes, CR and XT. The CR mode needs a simple
capacitor and resistor circuit attached to CLKIN, whose time constant (C ⫻ R)
determines the clock period. R should be between 3k and 100k, and C greater than 20pF.
For example, if R ⫽ 10kΩ and C ⫽ 10nF, the clock period will be around 2 ⫻ C ⫻
R ⫽ 200 μs (calculated from the CR rise/fall time) and the frequency about 5kHz. This
option is acceptable when the program timing is not critical.
The XT mode is the one most commonly used, since the extra component cost is small
compared with the cost of the chip itself and accurate timing is often a necessity. An
external crystal and two capacitors are fitted to CLKIN and CLKOUT pins. The crystal
frequency in this mode can be from 200kHz to 4MHz and is typically accurate to better
than 50ppm (parts per million) or 0.005%. A convenient value is 4Mz, as this is the
maximum frequency possible with a standard crystal and gives an instruction execution
time of 1.000 μs (1 million instructions per second, or 1Mip).
A low-speed crystal can be used to reduce power consumption, which is proportional to
clock speed in CMOS devices. The LP (low-power) mode supports the clock frequency
range 32–200kHz. To achieve the maximum clock speed of 20MHz, a high-speed (HS)
crystal is needed, with a corresponding increase in power consumption.
The MCU configuration fuses must be set to the required clock mode when the chip is
programmed. Many PIC chips now have an internal oscillator, which needs no external
components. It is more accurate than the RC clock but less accurate than a crystal. It
typically runs at 8MHz and can be calibrated in the chip configuration phase to provide a
more accurate timing source.
Configuration Options
Apart from the clock options, several other hardware options must be selected.
Watchdog Timer
When enabled, the watchdog timer (WDT) automatically resets the processor after a
given period (default 18ms). This allows, for example, an application to escape from
an endless loop caused by a program bug or run-time condition not anticipated by the
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software designer. To maintain normal operation, the WDT must be disabled or reset
within the program loop before the set time-out period has expired. It is therefore
important to set the MCU configuration bits to disable the WDT if it is not intended to
use this feature. Otherwise, the program is liable to misbehave, due to random resetting of
the MCU.
Power-up Timer
The power-up timer (PuT) provides a nominal 72ms delay between the power supply
voltage reaching the operating value and the start of program execution. This ensures
that the supply voltage is stable before the clock starts up. It is recommended that it be
enabled as a precaution, as there is no adverse effect on normal program execution.
Oscillator Start-up Timer
After the power-up timer has expired, a further delay allows the clock to stabilize before
program execution begins. When one of the crystal clock modes is selected, the CPU
waits 1024 cycles before the CPU is enabled.
Brown-out Reset (BoR)
It is possible for a transitory supply voltage drop, or brown-out, to disrupt the MCU
program execution. When enabled, the brown-out detection circuit holds the MCU in
reset while the supply voltage is below a given threshold and releases it when the supply
has recovered. In CCS C, a low-voltage detect function triggers an interrupt that allows
the program to be restarted in an orderly way.
Code Protection (CP)
The chip can be configured during programming to prevent the machine code being read
back from the chip to protect commercially valuable or secure code. Optionally, only
selected portions of the program code may be write protected (see WRT_X% later).
In-Circuit Programming and Debugging
Most PIC chips now support in-circuit programming and debugging (ICPD), which
allows the program code to be downloaded and tested in the target hardware, under the
control of the host system. This provides a final test stage after software simulation has
been used to eliminate most of the program bugs. MPLAB allows the same interface to be
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used for debugging in both the simulation and in-circuit modes. The slight disadvantage
of this option is that care must be taken that any application circuit connected to the
programming/ICPD pins does not interfere with the operation of these features. It is
preferable to leave these pins for the exclusive use of the ICPD system. In addition, a
small section of program memory is required to run the debugging code.
Low-Voltage Programming Mode
The low-voltage programming mode can be selected during programming so that
the customary high (12V) programming voltage is not needed, and the chip can be
programmed at Vdd (⫹5V). The downside is that the programming pin cannot then be
used for digital I/O. In any case, it is recommended here that the programming pins not
be used for I/O by the inexperienced designer, as hardware contention could occur.
Electrically Erasable Programmable Read Only Memory
Many PIC MCUs have a block of nonvolatile user memory where data can be stored
during power-down. These data could, for example, be the secure code for an electronic
lock or smart card reader. The electrically erasable programmable read only memory
(EEPROM) can be rewritten by individual location, unlike flash program ROM. The ‘877
has a block of 256 bytes, which is a fairly typical value. There is a special read/write
sequence to prevent accidental overwriting of the data.
Configuration in C
The preprocessor directive #fuses is used to set the configuration fuses in C programs
for PICs. A typical statement is
#fuses XT,PUT,NOWDT,NOPROTECT,NOBROWNOUT
The options defined in the standard CCS C 16F877 header file are
Clock Type Select LP, XT, HS, RC
Watchdog Timer Enable WDT, NOWDT
Power Up Timer Enable PUT, NOPUT
Program Code Protect PROTECT, NOPROTECT
In Circuit Debugging Enable DEBUG, NODEBUG
Brownout Reset Enable BROWNOUT, NOBROWNOUT
Low Voltage Program Enable LVP, NOLVP
EEPROM Write Protect CPD, NOCPD
Program Memory Write Protect WRT_50%, WRT_25%,
(with percentage protected) WRT_5%, NOWRT
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The default condition for the fuses if no such directive is included is equivalent to
#fuses RC,WDT,NOPUT,BROWNOUT,LVP,NOCPD,NOWRT
This corresponds to all the bits of configuration register being default high.
1.3 PIC16 MCU Peripherals
● Digital I/O
● Timers
● A/D converter
● Comparator
● Parallel slave port
● Interrupts
Basic digital input and output (I/O) in the microcontroller uses a bidirectional port
pin. The default pin configuration is generally digital input, as this is the safest option
if some error has been made in the external connections. To set the pin as output, the
corresponding data direction bit must be cleared in the port data direction register (e.g.,
TRISD). Note, however, that pins connected to the analog-to-digital (A/D) converter
default to the analog input mode.
The basic digital I/O hardware is illustrated in simplified form in Figure 1.4, with
provision for analog input. The 16 series reference manual shows equivalent circuits for
individual pins in more detail. For input, the current driver output is disabled by loading
the data direction bit with a 1, which switches off the tristate gate. Data are read into the
input data latch from the outside world when its control line is pulsed by the CPU in the
course of a port register read instruction. The data are then copied to the CPU working
register for processing.
When the port is set up for output, a 0 is loaded into the data direction bit, enabling the
current output. The output data are loaded into the data latch from the CPU. A data 1 at
the output allows the current driver to source up to 25mA at 5V, or whatever the supply
voltage is (2–6V). A data 0 allows the pin to sink a similar current at 0V.
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The 16F877 has the following digital I/O ports available:
Port A RA0–RA5 6 bits
Port B RB0–RB7 8 bits
Port C RC0–RC7 8 bits
Port D RD0–RD7 8 bits
Port E RE0–RE2 3 bits
Total digital I/O available 33 pins
Most of the pins have alternate functions, which are described later.
Timers
Most microcontrollers provide hardware binary counters that allow a time interval
measurement or count to be carried out separately from program execution. For example,
a fixed period output pulse train can be generated while the program continues with
another task. The features of the timers found in the typical PIC chip are represented in
Figure 1.5, but none of those in the ‘877 has all the features shown.
The count register most commonly is operated by driving it from the internal instruction
clock to form a timer. This signal runs at one quarter of the clock frequency; that is, one
instruction takes four cycles to execute. Therefore, with a 4-MHz clock, the timer counts
in microseconds (1-MHz instruction clock). The number of bits in the timer (8 or 16)
Data
Direction
Latch
Output
Data
Latch
Input
Data
Latch
Output
Current
Driver
Tristate
Output
Enable
Write TRIS bit
CPU Data Bus
Write Data bit
Read Data bit
Analog Input
Multiplexer
Figure 1.4: I/O Pin Operation
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determines the maximum count (256 or 65536, respectively). When the timer register
overflows and returns to zero, an overflow flag bit is set. This flag can be polled (tested)
to check if an overflow has occurred or an interrupt generated, to trigger the required
action.
To modify the count period, the timer register can be preloaded with a given number.
For example, if an 8-bit register is preloaded with the value 156, a time-out occurs after
256 ⫺ 156 ⫽ 100 clocks. Many timer modules allow automatic preloading each time
it is restarted, in which case the required value is stored in a preload register during timer
initialization.
A prescaler typically allows the timer input frequency to be divided by 2, 4, 8, 16, 32,
64, or 128. This extends the maximum count proportionately but at the expense of timer
precision. For example, the 8-bit timer driven at 1MHz with a prescale value of 4 counts
up to 256 ⫻ 4 ⫽ 1024 μs, at 4 μs per bit. A postscaler has a similar effect, connected at
the output of the counter.
In the compare mode, a separate period register stores a value that is compared with the
current count after each clock and the status flag set when they match. This is a more
elegant method of modifying the time-out period, which can be used in generating a pulse
width modulated (PWM) output. A typical application is to control the output power to
a current load, such as a small DC motor—more on this later. In the capture mode, the
timer count is captured (copied to another register) at the point in time when an external
signal changes at one of the MCU pins. This can be used to measure the length of an
input pulse or the period of a waveform.
The ’877 has three counter/timer registers. Timer0 has an 8-bit counter and 8-bit
prescaler. It can be clocked from the instruction clock or an external signal applied to
RA4. The prescaler can also be used to extend the watchdog timer interval (see later),
in which case it is not available for use with Timer0. Timer1 has a 16-bit counter and
prescaler and can be clocked internally or externally as per Timer0. It offers capture and
Clock
Source
Select
Prescaler
(Clock
Divide)
Postscaler
(Output
Divide)
Timer
Overflow/
Time-out
(Interrupt)
Flag
Capture Signal Capture Register
Compare Register
Binary Counter
Match Flag
Instruction Clock
External Pulse
Figure 1.5: General Timer Operation
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compare modes of operation. Timer2 is another 8-bit counter but has both a prescaler and
postscaler (up to 1:16) and a compare register for period control.
Further details are provided in Interfacing PIC Microcontrollers by the author and the
MCU data books. When programming in C, only a limited knowledge of timer operation
is necessary, as the C functions generally take care of the details.
A/D Converter
Certain PIC pins can be set up as inputs to an analog-to-digital converter (ADC). The
’877 has eight analog inputs, which are connected to Port A and Port E. When used
in this mode, they are referred to as AD0–AD7. The necessary control registers are
initialized in CCS C using a set of functions that allow the ADC operating mode and
inputs to be selected. An additional “device” directive at the top of the program sets the
ADC resolution. An analog voltage presented at the input is then converted to binary and
the value assigned to an integer variable when the function to read the ADC is invoked.
The default input range is set by the supply (nominally 0–5V). If a battery supply is used
(which drops over time) or additional accuracy is needed, a separate reference voltage
can be fed in at AN2 (⫹Vref) and optionally AN3 (–Vref). If only ⫹Vref is used, the
lower limit remains 0V, while the upper is set by the reference voltage. This is typically
supplied using a zener diode and voltage divider. The 2.56V derived from a 2V7 zener
gives a conversion factor of 10mV per bit for an 8-bit conversion. For a 10-bit input,
a reference of 4.096V might be convenient, giving a resolution of 4mV per bit. The
essentials of ADC operation are illustrated in Figure 1.6.
Comparator
The comparator (Figure 1.7) is an alternative type of analog input found in some
microcontrollers, such as the 16F917 used in the mechatronics board described later.
Figure 1.6: ADC Operation
Multiplexer
Input
Volts
0-Vf
Setup ADC
Read ADC
8-bit or 16-bit
Integer Result
Analog-
to-Digital
Converter
ANx
⫹Vref
Analog
Inputs
Reference Volts
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It compares the voltage at a pair of inputs, and a status bit is set if the C⫹ pin is higher
than C–. The comparator status bit may also be monitored at an output pin. The ’917
has two such comparator modules; they are enabled using a system function to set the
operating mode. The ’877 has no comparators, so the ADC must be used instead.
Parallel Slave Port
The parallel slave port on the ’877 chip is designed to allow parallel communications
with an external 8-bit system data bus or peripheral (Figure 1.8). Port D provides the
eight I/O data pins, and Port E three control lines: Read, Write, and Chip Select. If data
are to be input to the port, the pin data direction is set accordingly and data presented
to Port D. The chip select input must be set low and the data latched into the port data
register by taking the write line low. Conversely, data can be read from the port using the
read line. Either operation can initiate an interrupt.
Interrupts
Interrupts can be generated by various internal or external hardware events. They are
studied in more detail later in relation to programming peripheral operations. However,
at this stage, it is useful to have some idea about the interrupt options provided within the
MCU. Table 1.3 lists the devices that can be set up to generate an interrupt.
Comparator
Status Bit
Vc⫹ ⬎ Vc⫺
Vc⫹
Vc⫺
Figure 1.7: Comparator Operation
Interrupt
INTERNAL
Data ⫻ 8
EXTERNAL
Data ⫻ 8
Chip Select
Read
Write Parallel
Slave
Port
Figure 1.8: Parallel Slave Port Operation
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The most effective way of integrating timer operations into an application program is
by using a timer interrupt. Figure 1.9 shows a program sequence where a timer is run
to generate an output pulse interval. An interrupt routine (ISR) has been written and
assigned to the timer interrupt. The timer is set up during program initialization and
started by preloading or clearing it. The main program and timer count then proceed
concurrently, until a time-out occurs and the interrupt is generated. The main program
is suspended and the ISR executed. When finished, the main program is resumed at the
original point. If the ISR contains a statement to toggle an output bit, a square wave could
be obtained with a period of twice the timer delay.
When interrupts are used in assembly language programs, it is easier to predict the effect,
as the programmer has more direct control over the exact sequence of the ISR.
Table 1.3: Interrupts Sources in the PIC16F877
Interrupt Source Interrupt Trigger Event Interrupt Label
Timers
Timer0 Timer0 register overflow INT_TIMER0
Timer1 Timer1 register overflow INT_TIMER1
CCP1 Timer1 capture or compare detected INT_CCP1
Timer2 Timer2 register overflow INT_TIMER2
CCP2 Timer2 capture or compare detected INT_CCP2
Ports
RB0/INT pin Change on single pin RB0 INT_EXT
Port B pins Change on any of four pins, RB4–RB7 INT_RB
Parallel Slave Port Data received at PSP (write input active) INT_PSP
Analog Converter A/D conversion completed INT_AD
Analog Comparator Voltage compare true INT_COMP
Serial
UART Serial Port Received data available INT_RDA
UART Serial Port Transmit data buffer empty INT_TBE
SPI Serial Port Data transfer completed (read or write) INT_SSP
I2
C Serial Port Interface activity detected INT_SSP
I2
C Serial Port Bus collision detected INT_BUSCOL
Memory
EEPROM Nonvolatile data memory write complete INT_EEPROM
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A C program is generated automatically by the compiler, so the precise timing that results
from an interrupt is less obvious. For this reason, the use of a real-time operating system
(RTOS) is sometimes preferred in the C environment, especially when programs become
more complex. In fact, C was originally developed for precisely this purpose, to write
operating systems for computers. C interrupts are considered further in Section 3.2, and
RTOS principles are outlined in Section 5.4.
1.4 PIC16 Serial Interfaces
● USART asynchronous link
● SPI synchronous bus
● I2C synchronous bus
Serial data connections are useful because only one or two signal wires are needed,
compared with at least eight data lines for a parallel bus plus control signals. The typical
Program Execution
1
Start Counter
Statement
Program Execution
7
Continue
5
Time-out
Process
(Interrupt
Service
Routine)
3
Time-out
Interrupt
2
Run
Counter
until
Overflow
6
Return
from
Interrupt
4
Jump
to
ISR
Figure 1.9: Timer Interrupt Process
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PIC microcontroller offers a choice of serial interfaces. The best one for any given
communication channel depends on the distance between nodes, the speed, and the
number of hardware connections required.
USART
The universal synchronous/asynchronous receive transmit (USART) device is typically
used in asynchronous mode to implement off-board, one-to-one connections. The term
asynchronous means no separate clock signal is needed to time the data reception, so
only a data send, data receive, and ground wires are needed. It is quick and simple to
implement if a limited data bandwidth is acceptable.
A common application is connecting the PIC chip to a host PC for uploading data
acquired by the MCU subsystem (Figure 1.10). The USART link can send data up to 100
meters by converting the signal to higher-voltage levels (typically ⫾12V). The digital
signal is inverted and shifted to become bipolar (symmetrical about 0V, line negative
when inactive) for transmission.
The PIC 16F877 has a dedicated hardware RS232 port, but CCS C allows any pin to be
set up as an RS232 port, providing functions to generate the signals in software. The
basic form of the signal has 8 data bits and a stop and start bit. The bit period is set by
the baud rate. A typical value is 9600 baud, which is about 10k bits per second. The bit
period is then about 100 μs, about 1 byte per millisecond, or 1K byte per second.
Line
Driver
Interface
PIC MCU
TX1 Transmit
RX1 Receive
Ground Ground
HOST PC
COM PORT
RX2
TX2
⫹/⫺ 12V
Figure 1.10: USART Operation
Bit Period
Time
1
0
Idle Start
Bit
Stop
Bit
Bit
0
Bit
1
Bit
2
Bit
3
Bit
4
Bit
5
Bit
6
Bit
7
Figure 1.11: USART RS232 Signal
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The data are transferred between shift registers operating at the same bit rate; the receiver
has to be initialized to the same baud setting as the transmitter. Assuming we are looking
at TTL level data, in the idle state, the line is high. When it goes low, the receiver clock is
started, the data are sampled in the middle of each following data bit period, and data are
shifted into the receive register (Figure 1.11).
RS232 is used to access the standard serial LCD display, in which case, line drivers
are not necessarily required. ASCII characters and control codes are sent to operate the
display, which has its own MCU with a serial interface to receive and decode the data.
It then drives the pixel array to display alphanumeric characters. Most LCDs may also
be set up to display simple bit-mapped graphics. In simulation mode, an RS232 virtual
terminal provides a convenient way of generating alphanumeric input into the MCU for
testing. The ASCII codes are listed in Table 2.5.
Master
Serial Data Out, SDO
Serial Data In, SDI
Serial Clock, SCK
Slave Select
Outputs
SS1
SS2
SS3
Slave 1
SDO
SDI
SCK
!SS
Slave 2
SDO
SDI
SCK
!SS
Figure 1.12: SPI Connections
7 6 5 4 3 2 1 0 Data Bits
SDO/SDI
Clock
SCK
Figure 1.13: SPI Signals
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SPI Bus
The serial peripheral interface (SPI) bus provides high-speed synchronous data exchange
over relatively short distances (typically within a set of connected boards), using
a master/slave system with hardware slave selection (Figure 1.12). One processor must
act as a master, generating the clock. Others act as slaves, using the master clock for
timing the data send and receive. The slaves can be other microcontrollers or peripherals
with an SPI interface. The SPI signals are
● Serial Clock (SCK)
● Serial Data In (SDI)
● Serial Data Out (SDO)
● Slave Select (!SS)
To transfer data, the master selects a slave device to talk to, by taking its SS line low.
Eight data bits are then clocked in or out of the slave SPI shift register to or from the
master (Figure 1.13). No start and stop bits are necessary, and it is much faster than
RS232. The clock signal runs at the same speed as the master instruction clock, that is,
5MHz when the chip is running at the maximum 20MHz (16 series MCUs).
I2
C Bus
The interintegrated circuit (I2
C) bus is designed for short-range communication between
chips in the same system using a software addressing system. It requires only two signal
wires and operates like a simplified local area network. The basic form of the hardware
and data signal are illustrated in Figures 1.14 and 1.15.
The I2
C slave chips are attached to a two-wire bus, which is pulled up to logic 1 when
idle. Passive slave devices have their register or location addresses determined by a
combination of external input address code pins and fixed internal decoding. If several
memory devices are connected to the bus, they can be mapped into a continuous address
space. The master sends data in 8-bit blocks, with a synchronous clock pulse alongside
each bit. As for SPI, the clock is derived from the instruction clock, up to 5MHz at the
maximum clock rate of 20MHz.
To send a data byte, the master first sends a control code to set up the transfer, then the
8-bit or 10-bit address code, and finally the data. Each byte has a start and acknowledge
bit, and each byte must be acknowledged before the next is sent, to improve reliability.
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The sequence to read a single byte requires a total of 5 bytes to complete the process, 3 to
set the address, and 2 to return the data. Thus, a substantial software overhead is involved.
To alleviate this problem, data can be transferred in continuous blocks (memory page
read/write), which speeds up the transmission.
1.5 PIC16 MPLAB Projects
● MPLAB C Project
● Project Files
The PIC microcontroller program comprises a list of machine code instructions, decoded
and executed in sequence, resulting in data movement between registers, and arithmetic
and logic operations. MCU reset starts execution at address zero, and the instructions are
executed in address order until a program branch is decoded, at which point a new target
address is derived from the instruction. A decision is made to take the branch or continue
in sequence based on the result of a bit condition test. This process is described in detail
in PIC Microcontrollers by the author.
The program could be written in raw binary code, but this would require manual
interpretation of the instruction set. Therefore, the machine code is generated from
assembly code, where each instruction has a corresponding mnemonic form that is
7 6 5 4 3 2 1 0
SDA
SCL
Start
Address/Data Bits
Acknowledge
Figure 1.15: I2
C Signals
Figure 1.14: I2
C Connections
Master Slave 1 Slave 2 etc.
SDA
SCL
⫹5V
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more easily recognizable, such as MOVF05,W (move the data at Port A to the working
register). This low-level language is fine for relatively simple programs but becomes time
consuming for more complex programs. In addition, assembly language is specific to a
particular type of processor and, therefore, not “portable.” Another level of abstraction is
needed, requiring a high-level language.
C has become the universal language for microcontrollers. It allows the MCU
memory and peripherals to be controlled directly, while simplifying peripheral setup,
calculations, and other program functions. All computer languages need an agreed set
of programming language rules. The definitive C reference is The C Programming
Language by Kernighan and Ritchie, second edition, incorporating ANSI C standards,
published in 1983.
A processor-specific compiler converts the standard syntax into the machine code for a
particular processor. The compiler package may also provide a set of function libraries,
which implement the most commonly needed operations. There is variation between
compilers in the library function syntax, but the general rules are the same.
Usually, a choice of compilers is available for any given MCU family. Options for the
PIC at time of writing are Microchip’s own C18 compiler, Hi-Tech PICC, and CCS C.
CCS was selected for the current work because it is specifically designed for the PIC
MCU, supports the 16 series devices, and has a comprehensive set of peripheral driver
functions.
MPLAB C Project
The primary function of the compiler is to take a source text file PROJNAME.C and
convert it to machine code, PROJNAME.HEX. The hex file can then be downloaded
to the PIC MCU. The source file must be written in the correct form, observing the
conventions of both ANSI C and the specific compiler dialect. The first program we see
later in the tutorial section is shown in Listing 1.1.
This can be typed into any text editor, but we normally use the editor in MPLAB, the
standard Microchip development system software package. This provides file management,
compiler interface and debugging facilities for PIC projects, and may be downloaded free
of charge from www.microchip.com. Before starting work, the complier also has to be
installed. The compiler file path is set in MPLAB by selecting Project, Set Language Tool
Locations. The compiler can then be selected via the Project, Select Language Tool Suite
menu option. Browse for the compiler executable file (CCSC.EXE) and select it.
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A project folder called PROJNAME should now be created to hold the files that
will be generated and a new project created with the same name. A workspace window
appears with file project folders named Source Files, Header Files, and Other Files.
Open a new source window, type in the program header comment at the top of the
program as shown in Listing 1.1, and save the file as PROJNAME.C in the project
folder. Type the rest of the program in and save it. The source code must now be attached
to the project, by right clicking on Source Files workspace folder to open the “add file”
dialog.
Note, in the source code, a statement #include 16F877A.h. This defines the specific
chip for which the program is created and refers to a header file supplied with the
compiler. This file must be included because it holds information about the chip register
addresses, labeling, and so on (it can be viewed in any text editor and is listed in full in
Section 2.8). The file should be copied from the Devices folder in the CCS C program file
folder set into the project folder. It can then be attached to this project by right clicking
on the Header Files folder. We are now ready to compile the program by clicking on
the Compile button in the MPLAB main toolbar. The compiler execution dialog briefly
appears and, ideally, a “build succeeded” message is displayed.
The program can now be tested in simulation mode by selecting Debugger, Select Tool,
MPLAB SIM. This brings up a control panel in the main toolbar. Press Reset, and a
green arrow indicates the execution point at the top of the program. Run seems to have
little effect, but if View, Special Function Registers is selected, Port D can be seen to
have been written with the data FF. To see the program listed in assembler, select View,
Disassembler Listing. This shows an assembler version of the program derived from the
compiler output.
Listing 1.1 A Simple C Program
/*
OUTBYTE.C MPB 2-1-07 V1.0
*/
#include "16F877A.h" // MCU select
void main( ) // Main block
{
output_D(255); // Switch on outputs
}
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Project Files
Let us now look at some of files created in the project folder. Some, which are concerned
with MPLAB project management, do not need to be considered at this stage.
outbyte.c The source code file is created in a text edit window, in line with
the compiler and ANSI C syntax rules. For viewing outside MPLAB, it can be
“opened with” (right click) Notepad. The syntax requirements are detailed in the
C programming sections later.
outbyte.hex The hex file, the program download file, is shown in Listing 1.2,
as it is displayed in a text editor. The fact that it is readable shows that it is stored as
ASCII characters. It must be converted by the program downloading utility to actual
binary code for loading into program flash memory in the MCU. If the hex listing is
compared with the machine code column in the Disassembler listing visible in Figure
1.16, we can see that the first 4 bytes (eight digits) contain the start address 0000. The
program code starts at the ninth digit, but the bytes of the four-digit instruction code
are reversed. Therefore, the first instruction is code 3000 (MOVLW 0), but this is listed
in the hex file as 0030, indicating that, in program memory, the low byte is at the lower
(even) address, which is logical. The whole program is 40 bytes (80 hex digits), ending
at 6300 and highlighted in bold. Additional configuration data follow, and the file ends
with the MCU identifier.
outbyte.lst This contains the intermediate assembly language version of the
program, plus the configuration fuse settings. When viewed in a text window, it can be
seen that the configuration code is 3F73h, consistent with the program code.
outbyte.cof This file contains the machine code plus source file information that
allows debugging tools to display the source code and variables using their original labels.
This file is attached to the MCU in Proteus VSM to support source code debugging.
Listing 1.2 Program hex File
:1000000000308A0004280000840183131F30830518
:1000100083161F149F141F159F1107309C00880121
:08002000FF3083128800630029
:02400E00733FFE
:00000001FF
;PIC16F877A
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outbyte.err The error file provides debugging messages, which are displayed in
the Output, Build window after compilation.
outbyte.sym The symbol map shows the register locations in which the program
variables are stored.
outbyte.mcp This is the MPLAB project information file.
outbyte.mcw This is the MPLAB workspace information file.
outbyte.pjt This is the CCS compiler project information file.
1.6 PIC16 Program and Debug
● Programming the chip
● In-circuit debugging
● Design package
Figure 1.16: Screenshot of MPLAB Project
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Once the compiler has produced the hex file, it can be downloaded to the target
application board. However, it is generally preferable to test it first by software
simulation. This means running the program in a virtual MCU to test its logical function.
This can be done within MPLAB (tabular output) or using a third party debugging tool
such as Proteus VSM (graphical output). More details on simulation are provided in
Appendix C, and VSM interactive simulation is referred to throughout the text to provide
circuit schematics and debugging facilities.
Programming
A low-cost programmer available at the time of writing is the Microchip PICkit2
programmer (Figure 1.17). This connects to the USB port of the host PC, with the
programming module plugging direct into the target PCB. The six-way in-circuit
serial programming (ICSP) connector, between the programmer module and the target
board, must be designed into the application circuit. An in-line row of pins provides the
programmer connection to the target MCU, as shown in Figure 1.18.
Pin 1 carries the programming voltage (12–14V) and is connected to pin Vpp, which
doubles as the MCU reset input, !MCLR. Pin 4 (PGD) carries the program data and pin 5
(PGC), the program clock. Any other circuits connected to these pins must be designed
with care, so that they do not interfere with the programmer. The USB output provides
the target board power, up to a limit of 500mA, on pins 2 and 3. If necessary, a separate
target board supply must be provided.
Figure 1.17: PICkit2 Demo System Hardware (reproduced by permission of
Microchip Inc.)
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Once the hardware is connected up and the programmer drivers loaded, the programming
utility window (Figure 1.19) can be opened by running PICkit2.exe file, selected
from the Programmer menu. The hex file created by the compiler is imported via the file
menu and downloaded using the write button. The target program is run by checking the
On box.
Figure 1.19: PICkit2 Programmer Dialog
MCU
Vpp/!MCLR
Vdd
Vss
PGD
PGC
Reset
10k
Vdd Vss
Board ⫹5V Supply
ICSP
Interface
1
2
3
4
5
Figure 1.18: ICSP Target Board Connections
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Debugging
If in-circuit debugging is required, the Microchip MPLAB ICD2®
in-circuit debugger
(Figures 1.20 and 1.21) is recommended. This allows the application program to be
tested in real hardware by using the same MPLAB debugging tools used in the simulation
mode: source code display, run, stop, step, reset, breakpoints, and variable watch
windows. The target system needs its own power supply and an ICD connector.
With power supplied to the target, load the application project files. Select Debugger,
Select Tool, MPLAB ICD2. The debug control panel appears with controls to run, step,
and reset (Figure 1.22). If the program is recomplied after a change in the source code,
the target can be automatically reprogrammed.
Use of breakpoints is generally the most useful debugging technique in C, as it allows
complete blocks of assembler to be executed at full speed. These are enabled by right
clicking on the source code and indicated by a red marker. Once set, they can be
temporarily enabled and disabled. The watch window, selected from the View menu,
allows program variable values to be monitored as the program progresses.
Figure 1.20: Microchip ICD2 Module
PIC MCU
Target
System
ICD2
Interface
Host PC
MPLAB
Development
System
⫹ C Compiler
6-WAY
connector
USB
Figure 1.21: ICD2 Program and Debug System
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When debugging has been completed, the chip must be reprogrammed for the final time
by selecting Programmer, Select Tool, MPLAB ICD2. Then, hit the Program Target
Device button. When done, the program can be stopped and started using the Hold In
Reset and Release From Reset buttons. When the ICD pod is disconnected, the program
should auto-run in the target system.
Design Package
The components of the ECAD design package used in this book are listed below. The
PCB implementation tools are not described further, as they are outside the scope of this
programming guide.
● Circuit schematic capture (Proteus ISIS)
● Interactive circuit simulation (Proteus VSM)
● PCB layout design (Proteus ARES)
● PIC development system (Microchip MPLAB)
● PIC C Compiler (Custom Computer Services CCS C)
● PIC programming and in-circuit testing (Microchip ICD2)
Figure 1.22: ICD Debugging Windows
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Assessment 1
5 points each, total 100
1. List five consumer products that typically include a microcontroller.
2. Identify the five functional elements of a microcontroller.
3. Explain why flash ROM is an important technology in microcontrollers.
4. State five important characteristics of a microcontroller that should be
considered when selecting the best part for a given application.
5. Describe briefly the process of program execution in a microcontroller,
referring to the role of the program memory, instruction register, program
counter, file registers, and working register.
6. State the function of the following registers in the PIC16F877: 02h, 03h, 09h,
89h, 20h.
7. Explain the significance of the following abbreviations in relation to the
configuration of the PIC microcontroller: RC, XT, WDT, PUT, NOWRT.
8. Explain the function of the following elements of the PIC I/O circuit: tristate
gate, current driver, data direction latch, input data latch, output data latch.
9. A 16-bit PIC hardware timer is driven from the internal clock signal, and the
MCU is operating with a 20-MHz crystal. Calculate the preload value required
to produce an interrupt every 10ms.
10. If an analog-to-digital converter has a positive input reference voltage of
2.048V and is set up as for 8-bit conversion, calculate the resolution of the
ADC in millivolts per bit and the output code if the input voltage is 1.000V.
11. Refer to Figure 1.9, and briefly explain the timer interrupt process and why it is
useful.
12. Sketch the RS232 signal that transmits the character X (ASCII code 01011000)
on a line operating at ⫾12V. Indicate the stop and start bits as S and P.
13. Explain the difference between an asynchronous and synchronous data
transmission by reference to RS232 and SPI.
14. Explain the difference between hardware and software addressing as used by
SPI and I2
C.
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15. Explain briefly why SPI is generally faster than I2
C.
16. A page of plain text contains about 1000 ASCII characters. Estimate the
minimum time required to transmit this page over a 9600-baud RS232 link
and an SPI line, under the control of an MCU running at 20MHz, stating any
assumptions made.
17. State the function of each of the C project files that have the following
extension: C, HEX, COF, LST, and ERR.
18. State the function of the five connections in the PIC in-circuit programming and
debugging interface.
19. Study the content of the dissembler window in Figure 1.22, and state the
function of the five visible windows.
20. List a minimum set of development system hardware and software components
required to create a C application for the PIC microcontroller.
Assignments 1
Assignment 1.1
Download the data book for the PIC16F87X MCUs from www.microchip.com. Study
Figure 1.2, the PIC16F877 block diagram. Describe in detail the sequence of events that
occurs when the data code for 25510 (111111112) from a machine code instruction is
output to Port D. Refer to the role of the program memory, program counter, instruction
register, instruction decoder, file register addressing, internal data bus, and clock. What
path must the data follow to get from the program memory to Port C? Describe the setup
required in Port C to enable the data byte to be observed on the port pins (Figure 1.4).
Refer, if necessary, to PIC Microcontrollers: An Introduction to Microelectronics by the
author.
Assignment 1.2
Research a list of SPI and I2
C peripherals that might be useful in constructing PIC
applications. Identify typical memory, interfacing, and sensor chips that use these
interfaces and summarize the range of devices available for each interface.
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Assignment 1.3
Download and install MPLAB development system from www.microchip.com, and
the demo C complier for the PIC16F877 from www.ccsinfo.com. Create the project
OUTBYTE as described in Section 1.5. Enter the source code and save in the project
folder. Copy the header file into the same folder. Compile the program and view the files
created in the folder. Check that the .hex, .lst, and .cof files have been created. Test
the program in simulation mode; arrange the MPLAB windows as seen in Figure 1.6 and
check that Port C is loaded with the output byte FFh. Study the assembler version of the
program; note the number of instructions required to implement the C output statement.
Reset and step through the program, noting the two phases: initialization and loop.
Change the output number in the source code from 255 to 8510, recompile, and run. What
is the Port D output now in binary and hex?
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C Programming Essentials
2.1 PIC16 C Getting Started
● Simple program and test circuit
● Variables, looping, and decisions
● SIREN program
Programming PIC microcontrollers in C is introduced here using the simplest possible
programs, assuming that the reader has no previous experience of the language. The CCS
compiler uses ANSI standard syntax and structures. However, a compiler for any given
microcontroller uses its own variations for processor-specific operations, particularly
input and output processes. These are fundamental to MCU programs and so will be
introduced from the start.
Simple Program
Microcontroller programs contain three main features:
● Sequences of instructions
● Conditional repetition of sequences
● Selection of alternative sequences
The following basic programs show how these processes are implemented in CCS C. The
program in Listing 2.1 is a minimal program that simply sets the bits of an 8-bit port in
the 16F877 to any required combination.
P A R T 2
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The essential source code components can be identified. The include statement tells
the compiler to incorporate the header file for a particular MCU. It provides information
about the chip hardware features that the compiler needs to tailor the program. The
keywords void main indicate the start of the main program block, and the associated
braces (curly brackets) enclose the program statements. This program only contains one
statement, the function call output_D(nnn)that sends a binary code to Port D.
Program Creation
The development process was introduced in Part 1, and further details are provided in
Appendices A, B, and C. Briefly, the program project is created as follows:
1. Assuming that MPLAB and CCS C compiler are installed, create a folder for the
project files, and an MPLAB project called OUTNUM. Copy the MCU header
file 16F877.h from the CCS header file folder to the project folder.
2. Write the program (OUTNUM.C) in the source code edit window of MPLAB,
referring to the compiler manual for the correct syntax, and save it in the project
folder. Assign the source code and header file in the project window.
3. Build the project (compile and link all files) to create OUTNUM.COF. Correct
any syntax and linker errors.
4. Run the program in MPSIM simulation mode. Use the source code debugging
window to trace the program execution and the watch window to track the CPU
variables. Correct any logical errors.
5. Optionally, the program can be tested in Proteus VSM, which once installed, can
be selected from the debugger menu.
Listing 2.1 A Program to Output a Binary Code
// OUTNUM.C Outputs an 8-bit code at Port D in the 16F877 MCU
#include "16F877A.h" // MCU header file
void main() // Main block start
{
output_D(255); // Switch on outputs
}
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Program Testing
The program could be tested by downloading to a suitable hardware target system, but
it is preferable to debug it first in simulation mode, either in MPLAB or, preferably, in
Proteus VSM. In the VSM schematic capture and cosimulation module ISIS, the target
PIC is selected from the component library and placed on the schematic. The application
file OUTNUM.COF previously created by the compiler is attached to it (Figure 2.1) and
the schematic saved in the project folder. When the simulation is run, the state of the
outputs is indicated by red and blue indicators.
Although not absolutely necessary for program testing in simulation mode, a set of LEDs
with their load resistors are attached to Port D, since these are required in the actual
hardware to display the outputs (Figure 2.2). No other circuit components or connections
are required at this stage, since the simulation runs correctly without a clock circuit. In
the real hardware, the clock circuit must be added and !MCLR input tied to Vdd (5V).
Here, the clock frequency is set in the MCU properties dialog when the program is
attached. To take advantage of the full debugging facilities of MPLAB, Proteus VSM can
be run from within MPLAB by installing it in the debug tool menu. For this, a plug-in
needs to be downloaded from www.labcenter.co.uk. When selected, the simulator runs in
a VSM viewer window (Figure 2.3).
Figure 2.1: ISIS Dialog to Attach the Program
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RB0/INT
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26 1
2
20
19
3
4
5
6
7
8
9
10
9
8
7
6
5
4
3
2
1
19
20
21
22
27
28
29
30
18
17
16
15
14
13
12
11
RAO/ANO
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T10S0/T1CKl
RC1/T10Si/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2VREF
RA3/AN3/VREF
RA4/TOCKI
RA5/AN4/SS
REO/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/VPP/THV
14
1
2
3
4
5
6
7
8
9
10
Figure 2.2: OUTBYTE Test Circuit with Output LEDs
Program Analysis
The main program contains just one statement, output_D(255)
. This means output
the number 25510 as a binary code to Port D of the chip, setting all pins high (obviously,
any number between 0 and 255 results in a corresponding output bit combination). All
statements are terminated with a semicolon. This statement is a function call, which means
the compiler gets the machine code for this operation from the standard set of built-in
functions supplied with the compiler. This particular function is one of a set of library
functions of the form output_x(n), where x is the port number (A–E), and n is the output
value (0–255). The general form of the C function is function_name(). Any information
needed by the function, the function parameter(s), is inserted into the parentheses.
The main program starts with the key words void main() and is enclosed between
curly brackets, or braces, as they are officially known. All program blocks are enclosed
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by braces, allowing a multilevel hierarchical structure. Main is a special function that
contains the main program block, within which all lower-level functions are contained.
These can return a result to the calling function, but the keyword void preceding the
function name main means that this function returns no result, since it is the top-level
function.
The preprocessor directive #include 16F877A.h instructs the compiler to include
this processor-specific file at the top of the program. It contains labels for the registers in
the selected MCU, so that the compiler knows where to store MCU control variables.
Comments can be enclosed between slash/star (/*...*/) control characters or can
follow a double slash (//), in which case the comment is terminated with a line return.
The program header should contain as much information as possible to assist the user and
facilitate future modifications. Ideally, line comments should describe the effect of the
statement in the target system.
The meaning of the C program is independent of the layout on the page. Only the
sequence of characters is significant to the compiler. However, in practice, the program
source code should be arranged to make it as easy to understand as possible. Spaces or
tabs can be used to indent each block (program level), and the open and close braces
should be lined up in the same column so that the brace pairs can be matched up when
Figure 2.3: MPLAB IDE Screenshot
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checking the program. This makes subsequent source code debugging and modification
easier. The benefits of good layout become more obvious later, when more complex
programs are developed.
By tradition, C source code is written mainly in lower case, with upper case used for
certain key words.
2.2 PIC16 C Program Basics
● Variables
● Looping
● Decisions
The purpose of an embedded program is to read in data or control inputs, process them,
and operate the outputs as required. Input from parallel, serial, and analog ports are held
in the file registers for temporary storage and processing; and the results are output later
on, as data or a signal. The program for processing the data usually contains repetitive
loops and conditional branching, which depends on an input or calculated value.
Variables
Most programs need to process data in some way, and named variables are needed to hold
their values. A variable name is a label attached to the memory location where the variable
value is stored. When working in assembly language, a register label acts as the variable
name and has to be assigned explicitly. In C, the variable label is automatically assigned
to the next available location or locations (many variable types need more than 1 byte of
memory). The variable name and type must be declared at the start of the program block,
so that the compiler can allocate a corresponding set of locations. Variable values are
assumed to be in decimal by default; so if a value is given in hexadecimal in the source
code, it must be written with the prefix 0x, so that 0xFF represents 255, for example.
A variable called x is used in the program in Listing 2.2, VARI.C. Longer labels
are sometimes preferable, such as “output_value,” but spaces are not allowed. Only
alphanumeric characters (a–z, A–Z, 0–9) and underscore, instead of space, can be used.
By default, the CCS compiler is not case sensitive, so ‘a’ is the same as ‘A’ (even though
the ASCII code is different). A limited number of key words in C, such as main and
include, must not be used as variable names.
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The variable x is an 8-bit integer with whole number values 0–25510. The value in binary
can be seen when it is output at an 8-bit port. Generally, C integers (int) are stored as 16-
bit values, but C for 8-bit microcontrollers uses a default 8-bit integer format. In Program
VARI.C, an initial value is assigned to the variable (99), which is then used in the output
function. The point here is that the variable value can now be modified without having to
change the output function call itself.
In the program, an 8-bit variable x is declared and assigned a value 99 using the “equals”
operator. It is then output to Port D using the standard output function.
Looping
Most real-time applications need to execute continuously until the processor is turned
off or reset. Therefore, the program generally jumps back at the end to repeat the main
control loop. In C this can be implemented as a “while” loop, as in Listing 2.3.
The condition for continuing to repeat the block between the while braces is contained in
the parentheses following the while keyword. The block is executed if the value, or result of
the expression, in the parentheses is not zero. In this case, it is 1, which means the condition
is always true; and the loop repeats endlessly. This program represents in simple form the
general structure of embedded applications, where an initialization phase is followed by
an endless control loop. Within the loop, the value of x is incremented (x++). The output
Listing 2.2 Variables
/*
Source code file: VARI.C
Author, date, version: MPB 11-7-07 V1.0
Program function: Outputs an 8-bit variable
Simulation circuit: OUTBYTE.DSN
***************************************************************/
#include 16F877A.h
void main()
{
int x; // Declare variable and type
x=99; // Assign variable value
output_D(x); // Display the value in binary
}
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Listing 2.3 Endless Loop
// Source code file: ENDLESS.C
// Program function: Outputs variable count
#include 16F877A.h
void main()
{
int x; // Declare variable
while(1) // Loop endlessly
{ output_D(x); // Display value
x++; // Increment value
}
}
therefore appears to count up in binary when executing. When it reaches the maximum for
an 8-bit count (11111111 255), it rolls over to 0 and starts again.
Decision Making
The simplest way to illustrate basic decision making is to change an output depending on
the state of an input. A circuit for this is shown in Figure 2.4, INBIT.DSN. The switch
generates an input at RC0 and RD0 provides the test output.
The common keyword for selection in many high level languages is IF. Program IFIN.C
(Listing 2.4) has the usual endless “while” loop but contains a statement to switch off Port D
initially. The input state is read within the loop using the bit read function input(PIN_C0).
This assigns the input value 1 or 0 to the variable x. The value is then tested in the if
statement and the output set accordingly. Note that the test uses a double equals to differentiate
it from the assignment operator used in the previous statement. The effect of the program is
to switch on the output if the input is high. The switch needs to be closed before running to
see this effect. The LED cannot be switched off again until the program is restarted.
Loop Control
The program can be simplified by combining the input function with the condition
statement as follows:
if(input(PIN_C0))output_high(PIN_D0);
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Listing 2.4 IF Statement
// IFIN.C Tests an input
#include 16F877A.h
void main()
{
int x; // Declare variable
output_D(0); // Clear all outputs
while(1) // Loop always
{
x = input(PIN_C0); // Get input state
if(x==1)output_high(PIN_D0); // Change output
}
}
RB0/INT
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
D1
LED-RED
220R
R1
10k
22
27
28
29
30
RAO/ANO
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T10S0/T1CKl
RC1/T10Si/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2VREF
RA3/AN3/VREF
RA4/TOCKI
RA5/AN4/SS
REO/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/VPP/THV
14
1
2
3
4
5
6
7
8
9
10
Figure 2.4: INBIT.DSN Test Circuit with Input Switch
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Listing 2.5 Conditional Loop
// WHILOOP.C Input switch controls output flashing
#include 16F877A.h
#use delay (clock=1000000) // MCU clock = 1MHz
void main()
{
while(1)
{
while(input(PIN_C0)) // Repeat while switch open
{
output_high(PIN_D0);
delay_ms(300); // Delay 0.3s
output_low(PIN_D0);
delay_ms(500); // Delay 0.5s
}
output_low(PIN_D0); // Switch off LED
}
}
The conditional sequence can also be selected by a while condition. In Program
WHILOOP.C (Listing 2.5), the input is tested in the loop condition statement and the
output flashed on and off while the switch is open (input high). If the switch is closed,
the flash loop is not executed and the LED is switched off.
The program also demonstrates the delay function. If this were absent, the loop would
execute in just a few microseconds, since each machine code instruction takes 4μs at a
clock rate of 1MHz.The flashing of the output would be invisible. The delay required
(in milliseconds) is given as the function parameter, and a reference to the function
library is provided at the start of the program with the #use directive. This allows
the compiler to find the library routine delay_ms(). The clock speed of the target
processor must be given in the use directive, so that the correct delay is calculated
within the function.
Compare the syntax of the I/O statements. The function output_high(PIN_nn) is
an output operation to set the port pin high. The function input(PIN_nn) is an input
function that returns a 1 or 0 from the input pin, which can be tested by an IF or WHILE
statement. The ports are initialized automatically within these functions.
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FOR Loop
The WHILE loop repeats until some external event or internally modified value satisfies
the test condition. In other cases, we need a loop to repeat a fixed number of times. The
FOR loop uses a loop control variable, which is set to an initial value and modified for each
iteration while a defined condition is true. In the demo program FORLOOP.C (Listing 2.6),
the loop control parameters are given within the parentheses that follow the for keyword.
The loop control variable x is initially set to 0, and the loop continues while it is less than 6.
Value x is incremented each time round the loop. The effect is to flash the output five times.
The FORLOOP program also includes the use of the while loop to wait for the switch to
close before the flash sequence begins. In addition, an unconditional while loop terminates
the program, preventing the program execution from running into undefined locations
after the end of the sequence. This is advisable whenever the program does not run in a
continuous loop. Note that the use of the empty braces, which contain no code, is optional.
SIREN Program
A program combining some of these basic features is shown in SIREN.C (Listing 2.7).
This program outputs to a sounder rather than an LED, operating at a higher frequency.
Listing 2.6 FOR Loop
// FORLOOP.C Repeat loop a set number of times
#include 16F877A.h
#use delay (clock=1000000)
void main()
{
int x;
while(input(PIN_C0)){}; // Wait until switch closed
for (x=0; x5; x++) // For loop conditions
{
output_high(PIN_D0); // Flash sequence
delay_ms(500);
output_low(PIN_D0);
delay_ms(500);
}
while(1); // Wait for reset
}
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The delay is therefore in microseconds. The output is generated when the switch is closed
(input C0 low). The delay picks up the incrementing value of “step,” giving a longer pulse
each time the for loop is executed. This causes a burst of 255 pulses of increasing length
(reducing frequency), repeating while the input is on. Note that 255 is the maximum
value allowed for “step,” as it is an 8-bit variable. When run in VSM, the output can be
heard via the simulation host PC sound card. Note the inversion of the input test condition
using ! not true.
The header information is now more extensive, as would be the case in a real application.
Generally, the more complex a program, the more information is needed in the header.
Information about the author and program version and/or date, the compiler version, and
Listing 2.7 SIREN Program
/*
Source code file: SIREN.C
Author, date, version: MPB 11-7-07 V1.0
Program function: Outputs a siren sound
Simulation circuit: INBIT.DSN
Compiler: CCS C Version 4
***************************************************************/
#include 16F877A.h
#use delay (clock=1000000)
void main()
{
int step;
while(1) // Keep checking switch
{
while(!input(PIN_C0)) // Siren while switch ON
{
for(step=0;step255;step++) // Loop control
{
output_high(PIN_D0); // Sound sequence
delay_us(step);
output_low(PIN_D0);
delay_us(step);
}
}
}
}
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the intended target system are all useful. The program description is important, as this
summarizes the specification for the program.
Blank Program
A blank program is shown in Listing 2.8, which could be used as a general template. We
should try to be consistent in the header comment information, so a standard comment
block is suggested. Compiler directives are preceded by hash marks and placed before the
main block. Other initialization statements should precede the start of the main control
loop. Inclusion of the unconditional loop option while(1) assumes that the system will
run continuously until reset.
We now have enough vocabulary to write simple C programs for the PIC microcontroller.
A basic set of CCS C language components is shown in Table 2.1. Don’t forget the
semicolon at the end of each statement.
2.3 PIC16 C Data Operations
● Variable types
● Floating point numbers
● Characters
● Assignment operators
A main function of any computer program is to carry out calculations and other forms of
data processing. Data structures are made up of different types of numerical and character
variables, and a range of arithmetical and logical operations are needed. Microcontroller
programs do not generally need to process large volumes of data, but processing speed is
often important.
Variable Types
Variables are needed to store the data values used in the program. Variable labels are
attached to specific locations when they are declared at the beginning of the program,
so the MCU can locate the data required by each operation in the file registers.
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Table 2.1: A Basic Set of CCS C Source Code Components
C Compiler Directives
#include source files Include source code or header file
#use functions(parameters) Include library functions
C Program Block
main(condition){statements} Main program block
while(condition){statements} Conditional loop
if(condition){statements} Conditional sequence
for(condition){statements} Preset loop
CCS C Library Functions
delay_ms(nnn) Delay in milliseconds
delay_us(nnn) Delay in microseconds
output_x(n) Output 8-bit code at Port X
output_high(PIN_nn) Set output bit high
output_low(PIN_nn) Set output bit low
input(PIN_nn) Get input
Listing 2.8 Program Blank
// Source Code Filename :
// Author/Date/Version :
// Program Description :
// Hardware/simulation :
///////////////////////////////////////////////////////////////
#include 16F877A.h // Specify PIC MCU
#use // Include library routines
void main() // Start main block
{
int // Declare global variables
while(1) // Start control loop
{
// Program statements
}
} // End main block
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Integers
We have seen the integer (whole number) variable in use. In the 8-bit MCU, the default type
is an unsigned 8-bit number, giving a range of values of 0–255. This obviously is inadequate
for many purposes, so 16- and 32-bit integer types are also needed (see Table 2.2). The range
of a number is determined by the number of different binary codes that can be represented.
If n is the number of bits, 2n
different codes are possible. As 0 must be included, the highest
number is 2(n1)
. Hence, the 16-bit unsigned integer has the range 0–65535 (216
1) and the
32 bit 0–4294967295 (232
1). There is also a 1-bit type for bit storage.
Signed Integers
The signed integer uses the most significant bit (MSB) as the sign bit, so the range is
accordingly reduced by half. MSB 0 represents a positive number, MSB 1 indicates a
negative number. Therefore, the range for a 16-bit signed integer is –32767 to +32767.
The sign bit must be processed separately to get the right answer from a calculation.
Floating Point
Integers can represent only a limited range of numbers, with a precision of 0.5.
Therefore, the floating point (FP) type should be used for many calculations, particularly
those with a fractional result. The 32-bit FP format can represent decimal numbers from
about 1039
to 1038
, with a precision of about 107
(0.0000001). The number is stored
in exponential format, as used in a standard calculator. Twenty-three bits are used for the
significant digits, called the mantissa. Eight bits are used for the exponent part and one
Table 2.2: Range of Integer Variables
Name Type Minimum Maximum Range
int1 1 bit 0 1 1 20
unsigned int8 8 bits 0 255 256 28
signed int8 8 bits 127 127 256 28
unsigned int16 16 bits 0 65535 65536 216
signed int16 16 bits 32767 32767 65536 216
unsigned int32 32 bits 0 4294967295 4294967296 232
signed int32 32 bits 2147483647 2147483647 4294967296 232
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for the sign. The IEEE standard form has the sign bit as the MSB, but Microchip and
CCS use a slightly more logical form, where the sign bit is the MSB of the third byte,
leaving the exponent to be represented by the complete high byte (Table 2.3).
The significant figures of the floating point number (mantissa) are represented by a
positive fractional binary number whose value is between 0 and 1. As in any binary
number, the weighting of the 23 bits is a power of 2 series but fractional, that is, ½, ¼, 1⁄8,
1⁄16, 1⁄32, 1⁄64, …, ½23
. The final fraction represents the resolution of the format, that is, the
smallest step in the number sequence:
1/223
= 1/8388608 ⬇ 0.0000001 = 10–7
Hence, 32-bit floating point numbers are precise to about seven decimal places. The final
result can therefore be quoted to six decimal places, assuming that rounding errors are not
significant.
An example of a floating point number is given in Table 2.4. Its value can be determined
by following the process of conversion that comes next, which is the easiest way to
describe the FP format.
The 32-bit FP number given is
1000 0011 1101 0010 0000 0000 0000 0000
Table 2.3: Microchip/CCS Floating Point Number Format
Exponent Sign Mantissa
eeee eeee s mmm mmmm mmmm mmmm mmmm mmmm
8 bits 1 bit 23 bits
Table 2.4: Example of 32-Bit Floating Point Number Format
FP number: 1000 0011 1101 0010 0000 0000 0000 0000
Mantissa: 101 0010 0000 0000 0000 0000
Exponent: 1000 0011
Sign: 1 negative number
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The mantissa is the low 23 bits, and the set bit weighting gives the value
1/2 + 1/8 + 1/64 = 0.5 + 0.125 + 0.015625 = 0.640625
Then, 1 is added to shift the decimal part into the range between 1.9999999 and 1.000000:
Decimal number = 1.640625
Signed result = 1.640625
The exponent is given by the high byte: 1000 0011 = 13110
This includes an offset of 127 to allow for positive and negative exponents, so we subtract
127 to obtain the corrected exponent: 131-127 = +4
The multiplier value is then calculated from the binary exponent: 2
+4
= 16
The final value is found by multiplying this by the mantissa result:
16 x-1.640625 = –26.25
The range of numbers that can be represented by the FP format can be estimated from the
exponent range:
Minimum exponent value: 2–127
⬇ 10–39
Maximum exponent value: 2128
⬇ 10+38
This is adequate for most purposes. The disadvantage of this format is there are always
slight rounding errors; so if an integer is converted to a FP number and back, it no longer
is exact. This is illustrated in Figure 2.5, where integer variables have been assigned
their maximum values in a demo program and are displayed in the watch window after
running in MPSIM. The integers are correct, but the discrepancy due to rounding errors
between the working value of the floating point number and the original can be seen to be
12.3456793 12.3456789 0.0000004.
One advantage of C is that the exact method of calculation is normally concealed
within the built-in functions and operations. However, we still need to use the most
appropriate numerical format, because the C compiler does not tell us if the right answer
is obtained from any given calculation. This is where simulation is useful in real-time
applications—we can check that the answers are correct before they are used to modify
control outputs in real hardware. The integer types and ranges available in CCS C are
shown in Table 2.1.
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Character Variable
Text characters are generally represented by ASCII codes (Table 2.5). The basic set of 7-
bit characters includes the upper and lower case letters and the numerals and punctuation
marks found on the standard computer keyboard. For example, capital (upper case) A is
1000001 (6510). The numeric characters run from 0x30 (0) to 0x39 (9), so to convert to
the actual number from ASCII, simply subtract 0x30. The character variable is indicated
in C source code in single quotes. For example the statement answer='Y'; will assign
the value 0x59 to the variable ‘answer’.
Assignment Operations
A range of arithmetic and logic operations are needed where single or paired operands are
processed. The result is assigned to one of the operand variables or a third variable.
Integers can be used for simple unsigned arithmetic operations, giving an exact result.
However, in general, floating point numbers must be used for signed calculations, but
remember there will be small errors. Logical operations must use integers, as the numbers
are processed bit by bit. A complete set of operators is listed in Table 2.6.
Figure 2.5: Variable Types Demo Program Screenshot
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Table 2.5: The 7-Bit ASCII Codes
Low Bits
High Bits
010 011 100 101 110 111
0000 Space 0 @ P ` p
0001 ! 1 A Q a q
0010 2 B R b r
0011 # 3 C S c s
0100 $ 4 D T d t
0101 % 5 E U e u
0110 6 F V f v
0111 ' 7 G W g w
1000 ( 8 H X h x
1001 ) 9 I Y i y
1010 * : J Z j z
1011 ; K [ k {
1100 , L l |
1101 - M ] m }
1110 . N ^ n ⬃
1111 / ? O _ o Del
Figure 2.6 shows the output of a test program that carries out some sample operations.
The results are shown in a watch window after running the program in MPSIM. The
8-bit integer operations give the correct output while the result is in range. The product
of the multiplication (mulbyte) is clearly incorrect, while the result of the integer division
(divbyte) is truncated. Floating point calculations are required in this case. The floating
point results show nine significant figures, but only four are valid for the addition and
subtraction, seven for the multiplication, and the division result is also correct only to
seven figures.
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Conditional Operations
Where a logical condition is tested in a while, if, or for statement, relational operators
are used. One variable is compared with a set value or another variable, and the block
is executed if the condition is true. The conditional operators are shown in Table 2.7.
Note that double equals is used in the relational test to distinguish it from the assignment
operator.
Table 2.6: Arithmetic and Logical Operations
Operation Operator Description Source Code Example Result
Single operand
Increment Add 1 to
integer
result = num1++; 0000 0000 0000
0001
Decrement -- Subtract 1
from integer
result = num1--; 1111 1111 1111
1110
Complement ⬃ Invert all
bits of
integer
result = ˜num1; 0101 0010 1010
1101
Arithmetic operation
Add Integer or
float
result = num1+num2; 0000 1010
+0000 0111
0001
0001
Subtract Integer or
float
result = num1-num2; 0000 1010
–0000 0011
0000
0111
Multiply * Integer or
float
result = num1*num2; 0000 1010
*0000 0011
0001
1110
Divide / Integer or
float
result = num1/num2; 0000 1100
/0000 0011
0000
0100
Logical operation
Logical AND Integer
bitwise
result= num1num2; 1001 0011
0111 0001
0001
0001
Logical OR | Integer
bitwise
result= num1|num2; 1001 0011
|0111 0001
1111
0011
Exclusive OR ^ Integer
bitwise
result= num1^num2; 1001 0011
^0111 0001
1110
0010
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Figure 2.6: Results of Sample Arithmetic and Logic Operations in
MPLAB Program Simulation
Table 2.7: Conditional Operators
Operation Symbol Example
Equal to == if(a==0) b=b+5;
Not equal to != if(a != 1) b=b+4;
Greater than if(a 2) b=b+3;
Less than if(a 3) b=b+2;
Greater than or equal to = if(a4) b=b+1;
Less than or equal to = if(a 5) b=b+0;
Sometimes, a conditional test needs to combine tests on several values. The tests can be
compounded by using logical operators, as follows:
AND condition: if((ab)(c=d))…
OR condition: if((ab)||(c=d))…
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(a) (b)
Condition
True?
Statement
Block
Statement
Block
Condition
True?
Figure 2.7: Comparison of (a) While and (b) Do..While Loop
2.4 PIC16 C Sequence Control
● While loops
● Break, continue, goto
● If, else, switch
Conditional branching operations are a basic feature of any program. These must be
properly organized so that the program structure is maintained and confusion avoided.
The program then is easy to understand and more readily modified and upgraded.
While Loops
The basic while(condition) provides a logical test at the start of a loop, and the
statement block is executed only if the condition is true. It may, however, be desirable
that the loop block be executed at least once, particularly if the test condition is affected
within the loop. This option is provided by the do..while(condition) syntax. The
difference between these alternatives is illustrated in Figure 2.7. The WHILE test occurs
before the block and the DO WHILE after.
The program DOWHILE shown in Listing 2.9 includes the same block of statements
contained within both types of loop. The WHILE block is not executed because the
loop control variable has been set to 0 and is never modified. By contrast, ‘count’ is
incremented within the DO WHILE loop before being tested, and the loop therefore is
executed.
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Listing 2.9 DOWHILE.C Contains Both Types of ‘While’ Loop
// DOWHILE.C
// Comparison of WHILE and DO WHILE loops
#include 16F877A.H
main()
{
int outbyte1=0;
int outbyte2=0;
int count;
// This loop is not executed ..............
count=0;
while (count!=0)
{ output_C(outbyte1);
outbyte1++;
count--;
}
// This loop is executed...................
count=0;
do
{ output_C(outbyte2);
outbyte2++;
count--;
} while (count!=0);
while(1){};
}
Break, Continue, and Goto
It may sometimes be necessary to break the execution of a loop or block in the middle of
its sequence (Figure 2.8). The block must be exited in an orderly way, and it is useful to
have the option of restarting the block (continue) or proceeding to the next one (break).
Occasionally, an unconditional jump may be needed, but this should be regarded as a last
resort, as it tends to threaten the program stability. It is achieved by assigning a label to
the jump destination and executing a goto..label.
The use of these control statements is illustrated in Listing 2.10. The events that trigger
break and continue are asynchronous (independent of the program timing) inputs from
external switches, which allows the counting loop to be quit or restarted at any time.
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Listing 2.10 Continue, Break, and Goto
// CONTINUE.C
// Continue, break, and goto jumps
#include 16F877A.H
#use delay(clock=4000000)
main()
{
int outbyte;
again: outbyte=0; // Destination of goto
while(1)
{
output_C(outbyte); // Foreground operation
delay_ms(10);
outbyte++; // Increments Port C
if (!input(PIN_D0)) continue; // Skip other tests if input 0
low
if (!input(PIN_D1)) break; // Terminate loop if input 1 low
delay_ms(100); // Debounce inputs
if (outbyte==100) goto again; // Restart at 100
}
}
Label
Continue
Goto
Break
Statement
Block
Figure 2.8: Break, Continue, and Goto
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(a)
Condition
True?
If
Block
(b)
Condition
True?
If
Block
Else
Block
Figure 2.9: Comparison of (a) If and (b) If..Else
The goto again is triggered by the count reaching a set value, which could be better
achieved by using the While condition. In a more complex program, exiting a function in
this way risks disrupting program control, since the function is not properly terminated.
The significance of this should become clearer when functions are analyzed later.
If..Else and Switch..Case
We have seen the basic if control option, which allows a block to be executed or skipped
conditionally. The else option allows an alternate sequence to be executed, when the
if block is skipped. We also need a multichoice selection, which is provided by the
switch..case syntax. This tests a variable value and provides a set of alternative
sequences, one of which is selected depending on the test result.
These options are illustrated in flowchart form in Figures 2.9 and 2.10, and the if..
else and switch..case syntax is shown in Listing 2.11. The control statement
switch(variable)tests the value of the variable used to select the option block. The
keyword case n: is used to specify the value for each option. Note that each option
block must be terminated with break, which causes the remaining blocks to be skipped.
A default block is executed if none of the options is taken.
The same effect can be achieved using if..else, but switch..case provides a more
elegant solution for implementing multichoice operations, such as menus. If the case
options comprise more than one statement, they are best implemented using a function
block call, as explained in the next section.
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Test Variable Value
Default Procedure
Value 1? Procedure 1
Value 2? Procedure 2
Value 3? Procedure 3
Value n? Procedure n
Figure 2.10: Switch..Case Branching Structure
2.5 PIC16 C Functions and Structure
● Program structure
● Functions, arguments
● Global and local variables
The structure of a C program is created using functions (Figure 2.11). This is a block of
code written and executed as a self-contained process, receiving the required parameters
(data to be processed) from the calling function and returning results to it. Main()is the
primary function in all C programs, within which the rest of the program is constructed.
When running on a PC, main() is called by the operating system, and control is returned
to the OS when the C program is terminated. In the microcontroller, main() is simply
used to indicate the start of the main control sequence, and more care needs to be taken in
terminating the program. Normally, the program runs in a continuous loop, but if not, the
final statement should be while(1);, which causes the program to wait and prevents
the program running into undefined locations following the application code.
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Listing 2.11 Comparison of Switch and If..Else Control
// SWITCH.C
// Switch and if..else sequence control
// Same result from both sequences
///////////////////////////////////////////////////////////////
#include 16F877A.h
void main()
{
int8 inbits;
while(1)
{
inbits = input_D(); // Read input byte
switch(inbits) // Test input byte
{
case 1: output_C(1); // Input = 0x01, output = 0x01
break; // Quit block
case 2: output_C(3); // Input = 0x02, output = 0x03
break; // Quit block
case 3: output_C(7); // Input = 0x03, output = 0x07
break; // Quit block
default:output_C(0); // If none, output=0x00
}
if (input(PIN_D0)) output_C(1); // This block has same effect
if (input(PIN_D1)) output_C(2);
if (input(PIN_D0) input(PIN_D1)) output_C(7);
else output_C(0);
}
}
We have already seen built-in functions such as input(PIN_D0) and output_C(255),
which read and write the ports. Function “arguments,” given in the parentheses, allow
function parameters to be passed to the function block, in this case specifying the port or
pin to be accessed. Another example is delay_ms(100), which passes the required
delay time to the delay function.
In this case, the function code must be called up explicitly with the #use delay
(clock=4000000) directive. This tells the compiler to include the delay library
functions, allowing the system clock to be specified at the same time, so that the correct
delays can be calculated.
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LEVEL 0
void fun1()
{
}
Main()
{
}
statements
fun1()
statements
statements
....
....
....
....
statements
fun2(arg)
statements
statements
....
....
void fun3
{
statements
...
...
}
void fun2(arg)
{
}
statements
....
fun3
....
return(val)
LEVEL 1 LEVEL 2
Figure 2.11: Hierarchical C Program Structure
Basic Functions
A simple program using a function is shown in FUNC1.C, Listing 2.12. The main
block is very short, consisting of the function call out() and a while statement, which
provides the wait state at the end of main(). In this case, the variables are declared
before the main block. This makes them global in scope; that is, they are recognized
throughout the whole program and within all function blocks. The function out()is also
defined before main(), so that, when it is called, the function name is recognized. The
function starts with the keyword void, which indicates that no value is returned by the
function. The significance of this is explained shortly.
The function itself simply increments Port C from 0 to 255. It contains a for loop to
provide a delay, so that the output count is visible. This is a simple alternative to the
built-in delay functions seen in previous examples and is used here to avoid the inclusion
of such functions while we study user-defined functions. It simply counts up to a preset
value to waste time. The delay time is controlled by this set value.
For those readers familiar with assembly language, the disassembly listing for this
program is instructive. It will be seen that Call and Return are not used to implement the
function call. Instead, Goto is used throughout; this is to avoid the limited stack depth
(eight levels) in the PIC architecture, so that it is possible to have more that eight levels of
function calls in the program.
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Listing 2.12 Basic Function Call
// FUNC1.C
// Function call and program structure
///////////////////////////////////////////////////////////////
#include 16F877A.H
int8 outbyte=1; // Declare global variables
int16 n;
void out() ////////////////////////// Start of function block
{
while (outbyte!=0) // Start loop, quit when output 0
{ output_C(outbyte); // Output code 1 - 0xFF
outbyte++; // Increment output
for(n=1;n500;n++); // Delay so output is visible
}
}
main() ////////////////////////// Start of main block
{
out(); // Function call
while(1); // Wait until reset
}
Global and Local Variables
Now, assume that we wish to pass a value to the function for local use (that is, within the
function). The simplest way is to define it as a global variable, which makes it available
throughout the program. In program FUNC2.C, Listing 2.13, the variable count, holding
the delay count, hence the delay time, is global.
If there is no significant restriction on program memory, global variables may be used.
However, microcontrollers, by definition, have limited memory, so it is desirable to
use local variables whenever possible within the user functions. This is because local
variables exist only during function execution, and the locations used for them are freed
up on completion of function call. This can be confirmed by watching the values of C
program variables when the program is executed in simulation mode—the local ones
become undefined once the relevant function block is terminated.
If only global variables are used and the functions do not return results to the calling
block, they become procedures. Program FUNC3.C, Listing 2.14, shows how local
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Listing 2.13 Passing a Parameter to a Function
// FUNC2.C
// Uses global variables only
///////////////////////////////////////////////////////////////
#include 16F877A.H
int8 outbyte=1; // Declare global variables
int16 n,count;
void out() //////////////////// Function to run output count
{
while(outbyte!=0)
{ output_C(outbyte);
outbyte++;
for(n=1;ncount;n++); // Use global value for count
}
}
main() //////////////////// Main block
{
count=2000; // Set variable value
out(); // Call function
while(1); // Wait for reset
}
variables are used. The function out() runs a binary count, which is stopped when
a switch on pin D0 is closed. This value is then returned to the main program and
displayed. Variable n is local to function out()and is declared within the function.
Variable t is also local but receives its value from the variable count in the calling
routine. The value is transferred between the argument in the function call (count) and
the argument of the function declaration (int16 t). Note that the local integer type
must be declared in the function declaration. The function also returns a value outbyte
to the main block. This is displayed at Port C in the main routine.
2.6 PIC16 C Input and Output
● RS232 serial data
● Serial LCD
● Calculator and keypad
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Listing 2.14 Using Local Variables in Functions
// FUNC3.C
// Uses local variables
///////////////////////////////////////////////////////////////
#include 16F877A.H
int8 outbyte=1; // Declare global variables
int16 count;
int out(int16 t) ////////////// Declare argument types
{
int16 n; // Declare local variable
while (input(PIN_D0)) // Run at speed t
{ outbyte++;
for(n=1;nt;n++);
}
return outbyte; // Return output when loop stops
}
main() ////////////////////////////////////////////
{
count=50000;
out(count); // Pass count value to function
output_C(outbyte); // Display returned value
while(1);
}
If an electronic gadget has a small alphanumeric LCD, the chances are that it is a
microcontroller application. Smart card terminals, mobile phones, audio systems, coffee
machines, and many other small systems use this display. The LCD we use here has a
standard serial interface, and only one signal connection is needed. The signal format is
RS232, a simple low-speed protocol that allows 1 byte or character code to be sent at a
time. The data sequence also includes start and stop bits, and simple error checking can
be applied if required. The PIC 16F877, in common with many microcontrollers, has a
hardware RS232 port built in. Further details of RS232 are found elsewhere in this book.
Serial LCD
CCS C provides an RS232 driver routine that works with any I/O pin (that is, the
hardware port need not be used). This is possible because the process for generating
the RS232 data frame is not too complex and can be completed fast enough to generate
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the signal in real time. At the standard rate of 9600 baud, each bit is about 100μs long,
giving an overall frame time of about 1ms. The data can be an 8-bit integer or, more
often, a 7-bit ASCII character code. This method of transferring character codes via a
serial line was originally used in mainframe computer terminals to send keystrokes to the
computer and return the output—that is how long it’s been around.
In this example, the LCD receives character codes for a 2-row 16-character display. The
program uses library routines to generate the RS232 output, which are called up by the
directive #use RS232. The baud rate must be specified and the send (TX) and receive
(RX) pins specified as arguments of this directive. The directive must be preceded by a
#use delay, which specifies the clock rate in the target system. The LCD has its own
controller, which is compatible with the Hitachi 44780 MCU, the standard for this interface.
When the system is started, the LCD takes some time to initialize itself; its own MCU
needs time to get ready to receive data. A delay of about 500 ms should be allowed in
the main controller before attempting to access the LCD. A basic program for driving the
LCD is shown in Listing 2.15.
Characters are sent using the function call putc(code), whose argument is the ASCII
code for the character; the ASCII table given previously (Table 2.5) lists the available
codes. Note that the codes for ‘0’ to ‘9’ are 0x30 to 0x39, so conversion between the code
and the corresponding number is simple. Characters for display can be defined as ‘A’ to
‘Z’ and so on, in single quotes, in the program.
The character is then replaced by its code by the compiler. The display also needs control
codes, for example, to clear the display and reset the cursor to the start position after
characters have been printed. These are quoted as an integer decimal and sent as binary. Each
control code must be preceded by the code 254 (1111 1110) to distinguish it from data. The
code to start the second line of the display is 192. The display reverts automatically to data
mode after any control code. A basic set of control codes is identified in Table 2.8.
In the example program LCD.C, the sample character ‘acap’ is upper case ‘A’, ASCII
code 1000001 6510. If a string of fixed characters are to be displayed, the form
printf(sample text) can be used. The meaning of the function name is “print
formatted.” We often need to insert a variable value within fixed text; in this case, a
format code is placed within the display text, and the compiler replaces it with the value
of the variable, which is quoted at the end of the printf statement. The code %d means
display the variable value as an integer decimal number, %c means display the ASCII
character corresponding to the number. Multiple values can be inserted in order, as seen
in program LCD.C. A summary of formatting codes is shown in Table 2.9.
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Table 2.8: Essential Control Codes for Serial 2x16 LCD
Code Effect
254 Switch to control mode
followed by
00 Home to start of row 1
01 Clear screen
192 Go to start of row 2
Listing 2.15 Serial LCD Operation
// LCD.C
// Serial LCD test-send character using putc() and printf()
///////////////////////////////////////////////////////////////
#include 16F877A.h
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1) // Define speed
and pins
void main()
{
char acap='A'; // Test data
delay_ms(1000); // Wait for LCD to wake up
putc(254); putc(1); // Home cursor
delay_ms(10); // Wait for LCD to finish
while(1)
{
putc(acap); // Send test character
putc(254); putc(192); delay_ms(10); // Move to second row
printf(ASCII %c CHAR %d ,acap,acap); // Send test data again
while(1);
}
}
Listing 2.16 shows the program FLOAT.C, which illustrates how different variable types
are displayed, as well as showing the range of each type. Each variable type is output in
turn to the display. The general form of the format code is %nt, where n is the number
of significant figures to be displayed and t is the output variable type. The number of
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Table 2.9: Output Format Codes
Code Displays
%d Signed integer
%u Unsigned integer
%Lu Long unsigned integer (16 or 32 bits)
%Ls Long signed integer (16 or 32 bits)
%g Rounded decimal float (use decimal formatting)
%f Truncated decimal float (use decimal formatting)
%e Exponential form of float
%w Unsigned integer with decimal point inserted (use decimal formatting)
%X Hexadecimal
%LX Long hex
%c ASCII character corresponding to numerical value
%s Character or string
decimal places printed can also be specified for floating point numbers; for example,
%5.3d displays a decimal number with five significant digits and three decimal places.
Keypad and Calculator
A simple calculator application demonstrates the use of the LCD and a keypad, as well as
some numerical processing.
A matrix keypad provides a simple data entry device for microcontroller systems. The
keys are connected in rows and columns, such that pressing a button connects a row to a
column. The required connections are shown in Figure 2.12. The rows, labeled A, B, C,
and D, are connected as outputs at Port B, avoiding the programming pins. The columns,
labeled 1, 2, 3, and 4, are connected as inputs on Port D and are pulled up to 5V by
10-k resistors. A serial LCD, described previously, is driven from pin 7 of Port D.
To read the keypad, each row is set low in turn and the state of the inputs tested. If no
button is pressed, all the inputs remain high. When a key is operated, a low on that
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Listing 2.16 Formatted Variable Output to a Serial Display
/* FLOAT.C MPB 4307
Displays variable types and ranges
****************************************************/
#include 16F877A.h
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1)
int1 minbit=0, maxbit=1;
signed int8 minbyte=-127, maxbyte=127;
signed int16 minword=-32767, maxword=32767;
signed int32 minlong=-2147483647, maxlong=2147483647;
float testnum=12345.6789;
void main()
{
delay_ms(1000); // Wait for LCD to wake
putc(254); putc(1); // Home cursor
delay_ms(10); // Wait for LCD to do
while(1)
{
printf(Bit:%d or %d,minbit, maxbit); delay_ms(1000);
putc(254); putc(1); delay_ms(10);
printf(Byte %d to %d,minbyte, maxbyte); delay_ms(1000);
putc(254); putc(1); delay_ms(10);
printf(Word %Ld,minword); putc(254); putc(192);
delay_ms(10); printf( to %Ld,maxword); delay_ms(1000);
putc(254); putc(1); delay_ms(10);
printf(Long %Ld,minlong); putc(254); putc(192);
delay_ms(10); printf( to %Ld,maxlong); delay_ms(1000);
putc(254); putc(1); delay_ms(10);
printf(Float %5.4g,testnum); putc(254); putc(192);
delay_ms(10); printf(or %e, testnum); delay_ms(1000);
putc(254); putc(1); delay_ms(10);
}
}
row is detected on the column input for that key, which allows a corresponding code
to be generated. This is a binary number or ASCII code, as required by the particular
application. Program CALC.C (Listing 2.17) runs on this hardware and implements a
simple calculator with limited range.
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Listing 2.17 Calculator Program
/*
Source Code Filename: CALC.C
Author/Date/Version: MPB 21-12-07
Program Description: Calculator demo program
Hardware/simulation : CALC.DSN
**********************************************************************/
#include 16F877A.h
#use delay(clock=4000000)
#use rs232(baud=9600,xmit=PIN_D7,rcv=PIN_D0)
// Declare variables **************************************************
int akey, keynum, opcode, numofdigs, start;
int32 num1, num2, result, rem1, rem2, rem3, rem4;
int32 hunsdig, tensdig, onesdig;
int32 hunthous, tenthous, thous, hunds, tens, ones;
RB0/INT
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
RAO/ANO
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T10S0/T1CKl
RC1/T10Si/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2VREF
RA3/AN3/VREF
RA4/TOCKI
RA5/AN4/SS
REO/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/VPP/THV
14
1
2
3
4
5
6
7
8
9
10
7 8 9
4 5 6
1 2 3
ON
c 0
A
B
C
D
1
2
3
4
LCD1
MILFORD-2X16-BKP
VDD
RXD
VSS
Figure 2.12: Calculator Schematic
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// Declare functions **************************************************
void scankey(); // Read keypad
void makenum(); // Construct input decimal from keys
// MAIN PROGRAM: Get numbers calculate ********************************
void main()
{
for(;;)
{
// Get numbers ...................................................
delay_ms(500); putc(254); putc(1); delay_ms(10); // Clear display
numofdigs=onesdig=tensdig=hunsdig=0; akey=0x30;
do
{ scankey(); // Get first number
putc(akey);
if((akey=0x30)(akey=0x39)) makenum();
} while((akey=0x30)(akey=0x39));
num1=(onesdig+(tensdig*10)+(hunsdig*100)); // Calculate it
opcode=akey;
numofdigs=onesdig=tensdig=hunsdig=0; akey=0x30; // Get second number
do
{ scankey();
putc(akey);
if((akey=0x30)(akey=0x39)) makenum();
} while((akey=0x30)(akey=0x39));
num2=(onesdig+(tensdig*10)+(hunsdig*100)); // Calculate it
// Calculate result.............................................
if(opcode==0x2F) result=num1/num2;
if(opcode==0x2A) result=num1*num2;
if(opcode==0x2D) result=num1-num2;
if(opcode==0x2B) result=num1+num2;
//Calc result digits............................................
hunthous=result/100000; rem1=result-(hunthous*100000);
tenthous=rem1/10000; rem2=rem1-(tenthous*10000);
thous=rem2/1000; rem3=rem2-(thous*1000);
hunds=rem3/100; rem4=rem3-(hunds*100);
tens=rem4/10; ones=rem4-(tens*10);
// Display digits...............................................
start=0;
if(hunthous!=0){putc(hunthous+0x30);start=1;}
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if((tenthous!=0)||(start==1)){putc(tenthous+0x30); start=1;}
if((thous!=0)||(start==1)){putc(thous+0x30); start=1;}
if((hunds!=0)||(start==1)){putc(hunds+0x30); start=1;}
if((tens!=0)||(start==1)){putc(tens+0x30); start=1;}
if((ones!=0)||(start==1)){putc(ones+0x30); start=1;}
while(akey!=0xFF) scankey();
}
}
// PROCEDURE: Derive input digits *********************************
void makenum()
{
keynum=akey-0x30;
numofdigs++;
if(numofdigs==3)
{ hunsdig=tensdig; tensdig=onesdig; onesdig=keynum; }
if(numofdigs==2)
{ tensdig=onesdig; onesdig=keynum; }
if(numofdigs==1)
onesdig=keynum;
}
// PROCEDURE: Scans keypad attached to Port D *********************
void scankey()
{
akey=0;
while(akey==0)
{
output_b(255); output_low(PIN_B1);
if(!input(PIN_D1))
{akey = 0x37; delay_ms(50); while(!input(PIN_D1)){};}
if(!input(PIN_D2))
{akey = 0x38; delay_ms(50); while(!input(PIN_D2)){};}
if(!input(PIN_D3))
{akey = 0x39; delay_ms(50); while(!input(PIN_D3)){};}
if(!input(PIN_D4))
{akey = 0x2F; delay_ms(50); while(!input(PIN_D4)){};}
output_b(255); output_low(PIN_B2);
if(!input(PIN_D1))
{akey = 0x34; delay_ms(50); while(!input(PIN_D1)){};}
if(!input(PIN_D2))
{akey = 0x35; delay_ms(50); while(!input(PIN_D2)){};}
if(!input(PIN_D3))
{akey = 0x36; delay_ms(50); while(!input(PIN_D3)){};}
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if(!input(PIN_D4))
{akey = 0x2A; delay_ms(50); while(!input(PIN_D4)){};}
output_b(255); output_low(PIN_B4);
if(!input(PIN_D1))
{akey = 0x31; delay_ms(50); while(!input(PIN_D1)){};}
if(!input(PIN_D2))
{akey = 0x32; delay_ms(50); while(!input(PIN_D2)){};}
if(!input(PIN_D3))
{akey = 0x33; delay_ms(50); while(!input(PIN_D3)){};}
if(!input(PIN_D4))
{akey = 0x2D; delay_ms(50); while(!input(PIN_D4)){};}
output_b(255); output_low(PIN_B5);
if(!input(PIN_D1))
{akey = 0xFF; putc(254); putc(1); delay_ms(500);}
if(!input(PIN_D2))
{akey = 0x30; delay_ms(50); while(!input(PIN_D2)){};}
if(!input(PIN_D3))
{akey = 0x3D; delay_ms(50); while(!input(PIN_D3)){};}
if(!input(PIN_D4))
{akey = 0x2B; delay_ms(50); while(!input(PIN_D4)){};}
}
}
The program incorporates a procedure makenum() to generate a one-, two-, or three-
digit integer from the individual input digits and scankey() to read each keystroke.
The functions are declared as prototypes before main(); this allows the functions to be
defined after main(). This is sometimes more logical—the main block is designed first,
and the details within the functions developed afterward. The main block is a continuous
loop defined by the control statement for(;;). This unconditional for statement is
equivalent to while(1), the unconditional while loop. The main loop processes the input
and calculates the resulting digits.
2.7 PIC16 C More Data Types
● Arrays and strings
● Pointers and indirect addressing
● Enumeration
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The data in a C program may be most conveniently handled as sets of associated
variables. These occur more frequently as the program data becomes more complex, but
only the basics are mentioned here.
Arrays
Arrays are sets of variable values having the same type and meaning. For example, each
word in a text file is stored as a character array, a sequence of ASCII codes. This is also
referred to as a string. A numerical array might be a sequence of voltage readings from an
analog input in a test system or controller. The program ARRAYS.C (Listing 2.18) shows
how they can be created and displayed. The arrays are declared using a collective name
Listing 2.18 Numerical and Character Arrays
// ARRAYS.C
// Demo of numerical and string arrays
// Attach ARRAYS.COF to LCD.DSN to display
///////////////////////////////////////////////////////////
#include 16F877A.h
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1)
main()
{
int8 aval=0, n; // Declare single variables
int8 anum[10]; // Declare integer array
char astring[16]; // Declare character array
// Start LCD...............................................
delay_ms(1000);
putc(254); putc(1); delay_ms(10);
// Assign data to arrays...................................
for ( n=0; n10; n++ ) { anum[n]=aval; aval++; }
strcpy(astring,Hello!);
// Display data.............................................
for ( n=0; n10; n++ ) printf(%d,anum[n]);
putc(254); putc(192); delay_ms(10);
puts(astring);
while(1); // Wait
}
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and subscript placeholder (anum[10]and astring[16]), which instructs the compiler to
allocate a suitable set of locations in RAM. The variable type declaration determines how
many locations per value are needed.
The numerical array values are initialized using a for loop; a variable n, which
increments from 0 to 9, is used as loop counter and also as the array index value. The
character array values are assigned using the function strcpy() (string copy). Its
arguments are the target array name astring and the text in double quotes, which is
copied to the array. The end of the string is automatically terminated by a zero value,
creating a “null terminated string.” This allows the end of the message to be easily
detected by a receiving device.
The numerical data are displayed on our 16x2 LCD using printf(), again using a
for loop. The string is output in a different manner; the puts() (put string) function
is simpler than printf() and avoids the need to output each character separately, using
putc(). However, printf() is still more convenient for displaying a fixed string.
Table 2.10 shows the contents of the RAM file registers after the program ARRAYS has
executed. It can be seen that the numerical array data has been allocated to locations
0x21 to 0x2A inclusive in the GPRs, with the character data in locations 0x2D to 0x32
inclusive. The characters are displayed in the right column, converted from ASCII. The
single integers are seen in the locations 0x2B and 0x2C (final value 0x0A). The data
bytes can be accessed directly in these locations using indirect addressing operators.
Indirect Addressing Operators
C provides various ways of manipulating data in memory. Since there always seems to be
several ways to get the same result, this can be confusing for the beginner. If a variable
Table 2.10: MPLAB Display of Array Data in File Register
Address 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII
000 -- 00 38 1C 00 00 00 00 01 00 00 00 00 00 00 00 –.8..... ........
010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ........ ........
020 00 00 01 02 03 04 05 06 07 08 09 0A 0A 48 65 6C ........ .....Hel
030 6C 6F 21 00 00 00 00 00 00 00 00 00 00 32 00 18 lo!..... .....2..
040 20 20 20 39 14 0A 00 00 00 00 00 00 00 00 00 00 9....... ........
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is declared in C, the next available RAM location, or locations, is reserved for it by the
compiler. As we have seen, CCS C can assign 1 bit, 1 byte (integer or character),
2 bytes (integer), or 4 bytes (integer or float).
If we initially concentrate on byte storage, we can see that it consists of two associated
values, the address of the location and the contents of the location. When a variable is
declared, its label is assigned to the file RAM address by the compiler. When the variable
is used, this address is used to access the variable value.
Often, it is useful to be able to do this explicitly, and some functions require it. Therefore,
the operators address_of (, ampersand) and contents_of (*, star) are provided. These are
illustrated in Figure 2.13, a screenshot of demo program POINTS.C.
An 8-bit integer labeled num1 is declared and initialized to the value 123 (0x7B). Pointer
point1 is then assigned the address of num1 (File RAM address 0x21), and num2 is
assigned the contents of the address pointed to by point1 (0x7B). These values can be
seen in the watch window and file register window. The pointer (contents_of operator)
can be used for accessing a sequence of data words in memory by incrementing,
decrementing, or modifying the pointer variable. The address_of operator can be used
to obtain the address of the first item in the array.
Figure 2.13: Program POINTS.C Demonstrating Address_of and
Contents_of Operators
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These operators are useful for accessing data arrays and structures. Structures are sets
of data that contain different variable types mixed together, but as they are used more
extensively in data processing applications than real-time applications, they will not be
covered here.
Enumeration
Individual variables can be assigned an initial value when declared. If we wish to declare
a set of numbers that are continuous, as in ARRAYS.C, a convenient way is to use the
enumeration variable type (Figure 2.14). In its simplest form, it assigns incrementing
values to a set of labels. Optionally, the value can be set explicitly at any point in the list,
and the values increment from there.
Note that the label values are not initialized in the file registers, just created in the
complier memory. In the example ENUMER.C illustrated, the value of label mar(03)
only appears in memory at address 0x21 when assigned to the integer variable month.
2.8 PIC16 C Compiler Directives
● Include and use directives
● Header file listing and directives
Figure 2.14: Enumeration MPLAB Screenshot
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Compiler directives are typically used at the top of the program to set up compiler
options, control project components, define constant labels, and so on before the main
program is created. They are preceded by the hash symbol to distinguish them from other
types of statements and do not have a semicolon to end the line.
Program Directives
Examples using the directives encountered thus far follow—refer to the compiler
reference manual for the full range of options.
#include 16F877A.h
The include directive allows source code files to be included as though they had been
typed in by the user. In fact, any block of source code can be included in this way, and
the directive can thus be used to incorporate previously written reusable functions. The
header file referred to in this case provides the information needed by the complier to
create a program for a specific PIC chip.
#use delay(clock=4000000)
The ‘use’ directive allows library files to be included. As can be seen, additional
operating parameters may be needed so that the library function works correctly. The
clock frequency given here needs to be specified so that both software and hardware
timing loops can be correctly calculated.
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1)
In this directive, the parameters set the RS232 data (baud) rate and the MCU pins to be
used to transmit and receive the signal. This software serial driver allows any available
pin to be used.
Header File
A selection of the more commonly used directives are seen in the processor header file,
which must be included in every program. The file 16F877A.H is reproduced in full in
Listing 2.19.
The device directive selects the target processor, and can be followed by various options.
One that we use later is ADC=8, which sets the resolution of the analog input conversion.
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Listing 2.19 Header File 16F877A.H
//////// Standard Header file for the PIC16F877A device /////////
#device PIC16F877A
#nolist
//////// Program memory: 8192x14 Data RAM: 367 Stack: 8
//////// I/O: 33 Analog Pins: 8
//////// Data EEPROM: 256
//////// C Scratch area: 77 ID Location: 2000
//////// Fuses: LP,XT,HS,RC,NOWDT,WDT,NOPUT,PUT,PROTECT,DEBUG,NODEBUG
//////// Fuses: NOPROTECT,NOBROWNOUT,BROWNOUT,LVP,NOLVP,CPD,NOCPD,WRT_50%
//////// Fuses: NOWRT,WRT_25%,WRT_5%
////////
///////////////////////////////////////////////////////////////////////
//
// Discrete I/O Functions: SET_TRIS_x(), OUTPUT_x(), INPUT_x(),
// PORT_B_PULLUPS(), INPUT(),
// OUTPUT_LOW(), OUTPUT_HIGH(),
// OUTPUT_FLOAT(), OUTPUT_BIT()
//
// Constants used to identify pins in the above are:
#define PIN_A0 40 // Register 05, pin 0 (5x8)+0=40
#define PIN_A1 41 // Register 05, pin 1 (5x8)+1=41
#define PIN_A2 42 // Register 05, pin 2 (5x8)+2=42
#define PIN_A3 43 // Register 05, pin 3 etc
#define PIN_A4 44 // Register 05, pin 4
#define PIN_A5 45 // Register 05, pin 5
#define PIN_B0 48 // Register 06, pin 0 (6*8)+0=48
#define PIN_B1 49 // Register 06, pin 1 etc
#define PIN_B2 50 // Register 06, pin 2
#define PIN_B3 51 // Register 06, pin 3
#define PIN_B4 52 // Register 06, pin 4
#define PIN_B5 53 // Register 06, pin 5
#define PIN_B6 54 // Register 06, pin 6
#define PIN_B7 55 // Register 06, pin 7
#define PIN_C0 56 // Register 07, pin 0 (7*8)+0=56
#define PIN_C1 57 // Register 07, pin 1 etc
#define PIN_C2 58 // Register 07, pin 2
#define PIN_C3 59 // Register 07, pin 3
#define PIN_C4 60 // Register 07, pin 4
#define PIN_C5 61 // Register 07, pin 5
#define PIN_C6 62 // Register 07, pin 6
#define PIN_C7 63 // Register 07, pin 7
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#define PIN_D0 64 // Register 08, pin 0 (8*8)+0=64
#define PIN_D1 65 // Register 08, pin 1 etc
#define PIN_D2 66 // Register 08, pin 2
#define PIN_D3 67 // Register 08, pin 3
#define PIN_D4 68 // Register 08, pin 4
#define PIN_D5 69 // Register 08, pin 5
#define PIN_D6 70 // Register 08, pin 6
#define PIN_D7 71 // Register 08, pin 7
#define PIN_E0 72 // Register 09, pin 0 (9*8)+0=72
#define PIN_E1 73 // Register 09, pin 1 etc
#define PIN_E2 74 // Register 09, pin 2
//////////////////////////////////////////////////////// Useful defines
#define FALSE 0 // Logical state 0
#define TRUE 1 // Logical state 1
#define BYTE int // 8-bit value
#define BOOLEAN short int // 1-bit value
#define getc getch // Alternate names..
#define fgetc getch // ..for identical functions
#define getchar getch
#define putc putchar
#define fputc putchar
#define fgets gets
#define fputs puts
////////////////////////////////////////////////////////////// Control
// Control Functions: RESET_CPU(), SLEEP(), RESTART_CAUSE()
// Constants returned from RESTART_CAUSE() are:
#define WDT_FROM_SLEEP 0 // Watchdog timer has woken MCU from sleep
#define WDT_TIMEOUT 8 // Watchdog timer has caused reset
#define MCLR_FROM_SLEEP 16 // MCU has been woken by reset input
#define NORMAL_POWER_UP 24 // Normal power on reset has occurred
////////////////////////////////////////////////////////////// Timer 0
// Timer 0 (AKA RTCC)Functions: SETUP_COUNTERS() or SETUP_TIMER0(),
// SET_TIMER0() or SET_RTCC(),
// GET_TIMER0() or GET_RTCC()
// Constants used for SETUP_TIMER0() are:
#define RTCC_INTERNAL 0 // Use instruction clock
#define RTCC_EXT_L_TO_H 32 // Use T0CKI rising edge
#define RTCC_EXT_H_TO_L 48 // Use T0CKI falling edge
#define RTCC_DIV_1 8 // No prescale
#define RTCC_DIV_2 0 // Prescale divide by 2
#define RTCC_DIV_4 1 // Prescale divide by 4
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#define RTCC_DIV_8 2 // Prescale divide by 8
#define RTCC_DIV_16 3 // Prescale divide by 16
#define RTCC_DIV_32 4 // Prescale divide by 32
#define RTCC_DIV_64 5 // Prescale divide by 64
#define RTCC_DIV_128 6 // Prescale divide by 128
#define RTCC_DIV_256 7 // Prescale divide by 256
#define RTCC_8_BIT 0
// Constants used for SETUP_COUNTERS() are the above
// constants for the 1st param and the following for
// the 2nd param:
/////////////////////////////////////////////////////////////////// WDT
// Watch Dog Timer Functions: SETUP_WDT() or SETUP_COUNTERS() (see above)
// RESTART_WDT()
// Constants used for SETUP_WDT() are:
#define WDT_18MS 8 // Watchdog timer interval=18ms
#define WDT_36MS 9 // Watchdog timer interval=36ms
#define WDT_72MS 10 // Watchdog timer interval=72ms
#define WDT_144MS 11 // Watchdog timer interval=144ms
#define WDT_288MS 12 // Watchdog timer interval=288s
#define WDT_576MS 13 // Watchdog timer interval=576ms
#define WDT_1152MS 14 // Watchdog timer interval=1.15ms
#define WDT_2304MS 15 // Watchdog timer interval=2.30s
////////////////////////////////////////////////////////////// Timer1
// Timer 1 Functions: SETUP_TIMER_1, GET_TIMER1, SET_TIMER1
// Constants used for SETUP_TIMER_1() are:
// (or (via |) together constants from each group)
#define T1_DISABLED 0 // Switch off Timer 1
#define T1_INTERNAL 0x85 // Use instruction clock
#define T1_EXTERNAL 0x87 // Use T1CKI as clock input
#define T1_EXTERNAL_SYNC 0x83 // Synchronise T1CKI input
#define T1_CLK_OUT 8
#define T1_DIV_BY_1 0 // No prescale
#define T1_DIV_BY_2 0x10 // Prescale divide by 2
#define T1_DIV_BY_4 0x20 // Prescale divide by 4
#define T1_DIV_BY_8 0x30 // Prescale divide by 8
/////////////////////////////////////////////////////////////// Timer 2
// Timer 2 Functions: SETUP_TIMER_2, GET_TIMER2, SET_TIMER2
// Constants used for SETUP_TIMER_2() are:
#define T2_DISABLED 0 // No prescale
#define T2_DIV_BY_1 4 // Prescale divide by 2
#define T2_DIV_BY_4 5 // Prescale divide by 4
#define T2_DIV_BY_16 6 // Prescale divide by 16
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/////////////////////////////////////////////////////////////////// CCP
// CCP Functions: SETUP_CCPx, SET_PWMx_DUTY
// CCP Variables: CCP_x, CCP_x_LOW, CCP_x_HIGH
// Constants used for SETUP_CCPx() are:
#define CCP_OFF 0 // Disable CCPx
#define CCP_CAPTURE_FE 4 // Capture on falling edge of
CCPx input pin
#define CCP_CAPTURE_RE 5 // Capture on rising edge of
CCPx input pi
#define CCP_CAPTURE_DIV_4 6 // Capture every 4 pulses of
input
#define CCP_CAPTURE_DIV_16 7 // Capture every 16 pulses of
input
#define CCP_COMPARE_SET_ON_MATCH 8 // CCPx output pin goes high
when compare succeeds
#define CCP_COMPARE_CLR_ON_MATCH 9 // CCPx output pin goes low
when compare succeeds
#define CCP_COMPARE_INT 0xA // Generate an interrupt when
compare succeds
#define CCP_COMPARE_RESET_TIMER 0xB // Reset timer to zero when
compare succeeds
#define CCP_PWM 0xC // Enable Pulse Width
Modulation mode
#define CCP_PWM_PLUS_1 0x1c
#define CCP_PWM_PLUS_2 0x2c
#define CCP_PWM_PLUS_3 0x3c
long CCP_1;
#byte CCP_1 = 0x15 // Addresses of CCP1 registers
#byte CCP_1_LOW = 0x15
#byte CCP_1_HIGH= 0x16
long CCP_2;
#byte CCP_2 = 0x1B // Addresses of CCP2 registers
#byte CCP_2_LOW = 0x1B
#byte CCP_2_HIGH= 0x1C
/////////////////////////////////////////////////////////////////// PSP
// PSP Functions: SETUP_PSP, PSP_INPUT_FULL(), PSP_OUTPUT_FULL(),
// PSP_OVERFLOW(), INPUT_D(), OUTPUT_D()
// PSP Variables: PSP_DATA
// Constants used in SETUP_PSP() are:
#define PSP_ENABLED 0x10 // Enable Parallel Slave Port
#define PSP_DISABLED 0 // Disable Parallel Slave Port
#byte PSP_DATA= 8 // Address of PSP data register
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/////////////////////////////////////////////////////////////////// SPI
// SPI Functions: SETUP_SPI, SPI_WRITE, SPI_READ, SPI_DATA_IN
// Constants used in SETUP_SSP() are:
#define SPI_MASTER 0x20 // Select SPI master mode
#define SPI_SLAVE 0x24 // Select SPI slave mode
#define SPI_L_TO_H 0 // Strobe data on rising edge of
clock
#define SPI_H_TO_L 0x10 // Strobe data on falling edge of
clock
#define SPI_DIV_4 0 // Master mode clock divided by 4
#define SPI_CLK_DIV_16 1 // Master mode clock divided by 16
#define SPI_CLK_DIV_64 2 // Master mode clock divided by 64
#define SPI_CLK_T2 3 // Master mode clock source=Timer2/2
#fine SPI_SS_DISABLED 1 // Slave select input disabled
#define SPI_SAMPLE_AT_END 0x8000
#define SPI_XMIT_L_TO_H 0x4000
///////////////////////////////////////////////////////////////// UART
// Constants used in setup_uart() are:
// FALSE - Turn UART off
// TRUE - Turn UART on
#define UART_ADDRESS 2
#define UART_DATA 4
////////////////////////////////////////////////////////////////// COMP
// Comparator Variables: C1OUT, C2OUT
// Constants used in setup_comparators() are: (see 16F877 data
sheet, figure 12.1)
#define A0_A3_A1_A3 0xfff04 // Two common reference
comparators
#define A0_A3_A1_A2_OUT_ON_A4_A5 0xfcf03 // Two independent
comparators with outputs
#define A0_A3_A1_A3_OUT_ON_A4_A5 0xbcf05 // Two common reference
comparators with outputs
#define NC_NC_NC_NC 0x0ff07 // Comparator inputs
disconnected
#define A0_A3_A1_A2 0xfff02 // Two independent
comparators
#define A0_A3_NC_NC_OUT_ON_A4 0x9ef01 // One independent
comparator with output
#define A0_VR_A1_VR 0x3ff06 // Two comparators with
common internal reference
#define A3_VR_A2_VR 0xcff0e // Two comparators with
common internal reference
#bit C1OUT =0x9c.6
#bit C2OUT =0x9c.7
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////////////////////////////////////////////////////////////////// VREF
// Constants used in setup_vref() are:
//
#define VREF_LOW 0xa0 // Comparator reference voltage low
range 0–3.75V nominal
#define VREF_HIGH 0x80 // Comparator reference voltage high
range 1.25V-3.75V nominal
// Or (with |) the above with a number 0-15 (reference voltage
selection within range)
#define VREF_A2 0x40
////////////////////////////////////////////////////////////////// ADC
// ADC Functions: SETUP_ADC(), SETUP_ADC_PORTS() (aka SETUP_PORT_A),
// SET_ADC_CHANNEL(), READ_ADC()
//
// Constants used for SETUP_ADC() are: (Fosc=MCU clock frequency)
#define ADC_OFF 0 // ADC Off
#define ADC_CLOCK_DIV_2 0x10000 // ADC clock=Fosc/2
#define ADC_CLOCK_DIV_4 0x4000 // ADC clock=Fosc/4
#define ADC_CLOCK_DIV_8 0x0040 // ADC clock=Fosc/8
#define ADC_CLOCK_DIV_16 0x4040 // ADC clock=Fosc/16
#define ADC_CLOCK_DIV_32 0x0080 // ADC clock=Fosc/32
#define ADC_CLOCK_DIV_64 0x4080 // ADC clock=Fosc/64
#define ADC_CLOCK_INTERNAL 0x00c0 // Internal 2-6us clock
// Constants used in SETUP_ADC_PORTS() are:
#define NO_ANALOGS 7 // None – all pins
are digital I/O
#define ALL_ANALOG 0 // A0 A1 A2 A3 A5 E0
E1 E2 are analog
#define AN0_AN1_AN2_AN4_AN5_AN6_AN7_VSS_VREF 1 // 7 analog, 1
reference input
#define AN0_AN1_AN2_AN3_AN4 2 // 5 analog, 3
digital I/O
#define AN0_AN1_AN2_AN4_VSS_VREF 3 // 4 analogue, 1
reference input
#define AN0_AN1_AN3 4 // 3 analog, 5
digital I/O
#define AN0_AN1_VSS_VREF 5 // 2 analog, 1
reference input
#define AN0_AN1_AN4_AN5_AN6_AN7_VREF_VREF 0x08 // 6 analog, 2
reference inputs
#define AN0_AN1_AN2_AN3_AN4_AN5 0x09 // 6 analog, 2
digital I/O
#define AN0_AN1_AN2_AN4_AN5_VSS_VREF 0x0A // 5 analog, 1
reference input
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#define AN0_AN1_AN4_AN5_VREF_VREF 0x0B // 4 analog, 2 reference
inputs, 2 digital
#define AN0_AN1_AN4_VREF_VREF 0x0C // 3 analog, 2 reference
inputs, 3 digital
#define AN0_AN1_VREF_VREF 0x0D // 2 analog, 2 reference
inputs, 4 digital
#define AN0 0x0E // 1 analog, 7 digital
#define AN0_VREF_VREF 0x0F // 1 analog, 2 reference,
5 digital
// Constants used in READ_ADC() are:
#define ADC_START_AND_READ 7 // This is the default if
nothing is specified
#define ADC_START_ONLY 1
#define ADC_READ_ONLY 6
////////////////////////////////////////////////////////////////// INT
// Interrupt Functions: ENABLE_INTERRUPTS(), DISABLE_INTERRUPTS(),
// EXT_INT_EDGE()
// Constants used in EXT_INT_EDGE() are:
#define L_TO_H 0x40 // Interrupt on rising edge of external input
#define H_TO_L 0 // Interrupt on falling edge of external input
// Constants used in ENABLE/DISABLE_INTERRUPTS() are:
#define GLOBAL 0x0BC0 // Identify all interrupts
#define INT_RTCC 0x0B20 // Identify Timer0 overflow interrupt
#define INT_RB 0x0B08 // Identify Port B change interrupt
#define INT_EXT 0x0B10 // Identify RB0 external interrupt
#define INT_AD 0x8C40 // Identify ADC finished interrupt
#define INT_TBE 0x8C10 // Identify RS232 transmit done interrupt
#define INT_RDA 0x8C20 // Identify RS232 receive ready interrupt
#define INT_TIMER1 0x8C01 // Identify Timer1 overflow interrupt
#define INT_TIMER2 0x8C02 // Identify Timer2 overflow interrupt
#define INT_CCP1 0x8C04 // Identify Capture1 or Compare1 interrupt
#define INT_CCP2 0x8D01 // Identify Capture2 or Compare2 interrupt
#define INT_SSP 0x8C08 // Identify Synchronous Serial Port interrupt
#define INT_PSP 0x8C80 // Identify Parallel Slave Port interrupt
#define INT_BUSCOL 0x8D08 // Identify I2C Bus Collision interrupt
#define INT_EEPROM 0x8D10 // Identify EEPROM write completion interrupt
#define INT_TIMER0 0x0B20 // Identify Timer0 overflow interrupt
#define INT_COMP 0x8D40 // Identify Analog Comparator interrupt
#list
#device PIC16F877A
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#define PIN_A0 40
The define directive causes simple text replacement in the source code and is used
primarily for defining constants, that is, fixed values used in the program. As can be seen,
most of the header file consists of this directive. In the previous example, the compiler
replaces the text PIN_A0 with the number 4010, which specifies bit 0 of file register 5 in
the PIC register set (5x8=40). We can therefore deduce that the compiler identifies each bit
in the file registers by counting from zero (file register 0, bit 0) through all the registers. In
other cases, a setup code for loading into a control register is defined, as follows.
#define T1_INTERNAL 0x85
In the header file, the constant values are associated mainly with the chip hardware (e.g.,
I/O pin identification) or constants used in the CCS I/O functions. However, they can
also be used to specify alternate function names and to create a MACRO. This is a block of
replacement code, allowing a frequently used code sequence to be replaced with a macro
name. We use it later to simplify the LCD driver code.
#list, #nolist
These turn the C source code insertion within the assembler list file on and off. It is turned
off at the beginning of the header file to stop the source code window being filled with the
header file, then turned on again at the end to show the user source code, which follows.
#byte, #bit
These are used to specify the address to be used for a particular bit- or byte-sized variable.
Comments have been added to the header file in Listing 2.19 to clarify the function of some
directives. For more details on the meaning of the defined constants, refer to the MCU
data sheet and CCS Compiler Reference Manual. Generally, the constants are values to be
loaded into the control registers to set up a specific peripheral interface. Not all the options
available within the MCU control registers are available as C function options. If necessary,
control bits in the peripheral setup registers can be written directly, using the ‘contents_of’
operator. If a function needs more than one argument, the constants may be combined with
an OR operator (
), so that the active bits from more than one control code take effect.
2.9 PIC16 C Assembler Routines
● Reasons for using assembly language
● Insertion of assembler sequence
● Overview of assembly language
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The default programming language of any microprocessor or microcontroller is its own
assembly language. The syntax used for any given processor is determined by its internal
architecture and the machine code instructions that control it. Assembly language is the
first-level abstraction from machine code, where each instruction is represented by a
corresponding text mnemonic.
Program Compilation
When compiled, a C program is converted into assembler, then to machine code. We also
have seen that one C statement translates into a whole sequence of assembler instructions.
Since each C statement is independently translated into machine code, there is often
unnecessary duplication of instructions. For example, each time a port is accessed, the
required initialization is repeated. As a result, the assembler program derived from C
source code is always considerably longer than an equivalent assembler program that
performs these functions.
For this reason, many compilers contain optimization routines that try to minimize this
problem by analyzing the compiler code and eliminating redundant operations. For example,
when an I/O operation is converted, the compiler can check to see if the port is already
correctly initialized; if so, repetition of the initialization can be eliminated from the code.
Alternatively, sections of the program can be written directly in assembler. Not only is
the code more compact, the timing is more predictable and execution faster. Say that a
fast pulse waveform is to be generated by toggling a port bit. The maximum frequency
depends on the number of instructions on the output loop. If the sequence is implemented
in C code, a loop of two statements is required (Listing 2.20). This compiles into the code
seen in Listing 2.21, and we see that a sequence of nine assembler instructions is obtained.
Taking into account that the last instruction, GOTO 0x6b, takes two instruction cycles to
complete, the total loop time will be ten instructions. If the MCU is clocked at 4MHz, each
instruction takes 1μs and the whole loop, 10μs. The period of the output then is 100kHz.
Listing 2.20 C Code Fragment for Pulse Output Loop
while(1)
{ output_high(PIN_D0);
output_low(PIN_D0);
}
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Listing 2.21 Disassembled Code for Pulse Output Loop
25: while(1)
26: { output_high(PIN_D0);
006B 1008 BCF 0x8, 0
006C 1283 BCF 0x3, 0x5
006D 1408 BSF 0x8, 0
27: output_low(PIN_D0);
006E 1683 BSF 0x3, 0x5
006F 1008 BCF 0x8, 0
0070 1283 BCF 0x3, 0x5
0071 1008 BCF 0x8, 0
28: }
0072 1683 BSF 0x3, 0x5
0073 286B GOTO 0x6b
Note the redundancy in the sequence; the pin data direction setting is repeated in each
statement, where the file register bank is selected (BCF 0x3,0x5), and the direction bit is
cleared to 0 (BCF 0x8,0).
Assembler Block
The maximum output frequency of the pulse waveform can be increased by using a small
assembler block to toggle the output bit. A program is shown in Listing 2.22 that outputs
a pulse train when a button connected to RB0 input is pressed (active low). The main
program provides initialization of the button interrupt and an assembler block, which
outputs the signal in a loop that is as short as possible. The interrupt routine at the top of
the program is called when the button is not pressed (default condition), switching off the
output and waiting for the button to be pressed again to resume the output.
The start of the assembler block is identified by the #asm directive and terminated with
#endasm. All the code between these points must conform to the PIC assembler syntax
requirements (see Instruction Set, Table 2.11). The interrupt still works, even though
it is set up in C, because ultimately the interrupt control settings are the same in C and
assembler. Listing 2.23 disassembles the assembly block.
Note that the compiler automatically includes the necessary file register bank select
command to access the port data bits. Port B, bit 0, is then set, cleared, and the GOTO
takes the execution point straight back to the set instruction, giving a total loop time of
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Listing 2.22 C Source Code with Assembler Block
/*
Source code file: FAST.C
Author, date, version: MPB 19-10-07 V1.0
Program function: Demo of assembler block
Simulation circuit: ASSEM.DSN
***********************************************************/
#include 16F877A.h
#use delay(clock=4000000)
// ISR switches off output and waits for button ************
#int_ext
void isrext()
{ output_low(PIN_D0);
delay_ms(100);
while(input(PIN_B0));
}
// Main block initializes interrupt and waits for button ***
void main()
{
enable_interrupts(int_ext);
enable_interrupts(global);
ext_int_edge(L_TO_H);
// Assembler block outputs high speed pulse wave *******
#asm
Start:
BSF 8,0
BCF 8,0
GOTO Start
#endasm
} // End of source code **********************************
four instructions, or 4μs. The output therefore runs at 250kHz, 2.5 times faster than the
C loop shown in Listing 2.20. If the MCU clock is uprated to the maximum 20MHz, the
output frequency is 1.25MHz.
A screenshot of this program, FAST.C, under test in MPLAB with VSM debugging
is shown in Figure 2.15. The frequency of the output is displayed on the VSM virtual
counter/timer instrument.
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Table 2.11: PIC 16FXXX Instruction Set by Functional Groups
Operation Example
Move
Move data from F to W MOVF 0C,W
Move data from W to F MOVWF 0C
Move literal into W MOVLW
Register
Clear W (reset all bits and value to 0) CLRW
Clear F (reset all bits and value to 0) CLRF 0C
Decrement F (reduce by 1) DECF 0C
Increment F (increase by 1) INCF 0C
Swap the upper and lower four bits in F SWAPF 0C
Complement F value (invert all bits) COMF 0C
Rotate bits Left through Carry Flag RLF 0C
Rotate bits Right through Carry Flag RRF 0C
Clear (reset to 0) the bit specified (e.g., bit 3) BCF 0C,3
Set (to 1) the bit specified (e.g., bit 3) BSF 0C,3
Arithmetic
Add W to F ADDWF 0C
Add F to W ADDWF 0C,W
Add L to W ADDLW0F9
Subtract W from F SUBWF 0C
Subtract W from F, placing result in W SUBWF 0C,W
Subtract W from L, placing result in W SUBLW0F9
Logic
AND the bits of W and F, result in F ANDWF 0C
AND the bits of W and F, result in W ANDWF 0C,W
AND the bits of L and W, result in W ANDLW0F9
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Table 2.11: (Continued)
Operation Example
OR the bits of W and F, result in F IORWF 0C
OR the bits of W and F, result in W IORWF 0C,W
OR the bits of L and W, result in W IORLW0F9
Exclusive OR the bits of W and F, result in F XORWF 0C
Exclusive OR the bits of W and F, result in W XORWF 0C,W
Exclusive OR the bits of L and W XORLW0F9
Test and Skip
Test a bit in F and Skip next instruction if it is Clear ( 0) BTFSC 0C,3
Test a bit in F and Skip next instruction if it is Set ( 1) BTFSS 0C,3
Decrement F and Skip next Instruction if it is now 0 DECFSZ 0C
Increment F and Skip next Instruction if it is now 0 INCFSZ 0C
Jump
Go To a Labeled Line in the Program GOTO start
Jump to the Label at the start of a Subroutine CALLdelay
Return at the end of a Subroutine to the next instruction RETURN
Return at the end of a Subroutine with L in W RETLW 0F9
Return from Interrupt Service Routine to next instruction RETFIE
Control
No Operation, delay for 1 cycle NOP
Go into Standby Mode to save power SLEEP
Clear Watchdog Timer to prevent automatic reset CLRWDT
Load Port Data Direction Register from W* TRIS06
Load Option Control Register from W OPTION
Notes: The result of operations can generally be stored in W instead of the file register by adding ‘W’ to the
instruction. General Purpose Register 1, address 0C, represents all file registers (00–4F).
Literal value 0F9 represents all values 00–FF. Bit 3 is used to represent File Register Bits 0–7.
For MOVE instructions data are copied to the destination but retained in the source register.
F Any file register (specified by number or label), example is 0C.
W Working register.
L Literal value (follows instruction), example is 0F9.
* Use of these instructions not now recommended by manufacturer.
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Listing 2.23 Assembler Block Disassembled
29: // Assembler block outputs high speed pulse wave *******
30:
31: #asm
32:
33: Start:
34: BSF 8,0 006B 1283 BCF 0x3, 0x5
006C 1408 BSF 0x8, 0
35: BCF 8,0 006D 1008 BCF 0x8, 0
36: GOTO Start 006E 286C GOTO 0x6c
37:
38: #endasm
Figure 2.15: Debug Screenshot of FAST.C Showing Output Frequency
PIC Assembly Language
A complete introduction to programming PIC microcontrollers in assembly language
is given in PIC Microcontrollers, An Introduction to Microelectronics by the author
(Elsevier, second edition, 2004). A brief overview is given here for those readers
interested primarily in C programming. To program in assembler, some knowledge
of the internal hardware of the MCU is needed. The PIC16F877A architecture was
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introduced in Part 1 of this book, and the file register set is detailed further in
Appendix C.
The primary purpose of any programming language is to get data into a system, process
it, and output it in some useful form. In assembly language, the program statements act
directly on the MCU registers. All the hardware information needed for programming
in assembler is given in the data sheet for each PIC MCU, including the instruction set,
register details, and setup requirements.
A simplified version of the instruction set is shown in Table 2.11. It is organized by
function; that is, instructions with similar functions are grouped together. As explained in
Part 1, the operation of the MCU revolves around the numbered file register set and the
working register, designated W in the instructions. Register 0C (1210), the first general
purpose register, is used to represent the file registers in the examples. The special
function registers at the low addresses, which control the MCU setup and program
execution, are accessed in exactly the same way as the data registers.
The Move instructions are the most commonly used; these allow a data byte to be moved
from the working register to a file register and back or to load immediate data into W. Note
that data cannot be moved directly between file registers in the 16FXXX instruction set—this
is one of the casualties of the minimal instruction set (RISC) chip design philosophy. The
register instructions operate on a single file register, allowing it to be cleared, incremented,
decremented, rotated (shifted), and so on. Individual bits may also be set and cleared.
The Arithmetic and Logic instructions operate on pairs of registers in binary: adding,
subtracting, and carrying out logical bit-wise operations. If the result of an operation is
0 or a carry or overflow occurs, this is recorded in a flag bit in the status register
(SFR 03). For example, if a result is 0, the status register bit 2 is set. The flag can then
be used by a bit Test and Skip instruction to select alternate program sequences. In the
PIC, this is implemented by the instruction following the test being skipped or not,
depending on the result. Usually, this a jump instruction (GOTO or CALL), which takes
the program execution point to a new position (or not).
GOTO means go to a given program memory location unconditionally. CALL also means
jump but store a return address, so that the current sequence can be resumed when the
subroutine is finished, indicated by the RETURN instruction. The jump destination is
normally given a label, such as “start” in the example, in the source code.
Unlike C, the program designer must allocate memory explicitly, using suitable labels;
variables are declared using an equate directive at the top of the program to identify
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a GPR for that byte. The register labels are then recognized by the assembler as
representing a specific location. Obviously, only 8-bit variables can be used in assembler,
so care must be taken if using long values generated in the C program sections. An
assembler header file can allocate standard labels to the SFRs in the same way as the C
header defines the control register codes. The #include directive is the same in C and
assembler and can be used to include assembler header, library, and user source code.
There are only 35 core instructions in the 16FXXX instruction set. This reduced
instruction set increases the program execution speed. Additional special instructions
are available to compensate for the limited instruction set; these are basically predefined
macros. A macro is a code sequence that can be predefined and given its own name,
then inserted by the assembler when invoked by name. User-defined macros may also be
created as required.
Therefore, if direct control of the MCU registers and instruction sequence is required for any
reason or the speed of execution is critical, the C programmer can always revert to assembler
code. Since most microcontroller application designers are familiar with assembly language
anyway, including assembler blocks typically requires little additional learning time.
Assessment 2
(5 points each, total 100)
1. List the syntax features that a minimal C program must contain if compiled for
the PIC16F877A MCU.
2. List the steps required to create and test a C program for a PIC MCU prior to
downloading to hardware.
3. Write a C statement that outputs the 8-bit value 6410 to Port C. Write an
alternative 1-bit output statement that has the same effect, assuming all the port
bits are initially 0.
4. Describe briefly the difference between a WHILE loop, a DO..WHILE loop, and
a FOR loop.
5. Describe the effect of the following statements on active high LEDs connected to
Port D, assuming an active low switch circuit is connected to pin RC7:
output_D(255); delay_ms(1000);
while(!input(PIN_C7))
{output_D(15);}
output_D(0);
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6. Calculate the highest positive number that can be represented by the following
variable types: (a) 8-bit unsigned integer, (b) 16-bit signed integer, (c) 32-bit
floating point number.
7. Estimate the degree of precision provided by the following numerical types as a
percentage, to two significant figures: (a) 8-bit integer, (b) 32-bit FP number.
8. Work out the value of the FP number represented by the binary code
1000 0010 0011 0000 0000 0000 0000 0000
9. Write a C statement to convert numbers 0 to 9 to their ASCII hex code, using
variables ‘n’ for the number and ‘a’ for the ASCII code and send it to serial LCD.
10. State the result of each of these operations in decimal and 4-bit binary, if n=5
and m7:
(a) n++.
(b) ˜m.
(c) nm.
(d) n|m.
(e) n^m.
11. State the effect of the jump commands continue, break, and goto label
when used within a program loop.
12. A menu is required with a choice of three options to be selected by a numerical
variable x1, 2, 3. Each option is implemented in a separate function, funx().
Write a C code section to show how switch can be used to implement the menu.
13. Explain why the use of local variables is preferable in C programs designed for
microcontrollers with limited RAM.
14. Explain how the use of functions leads to well-structured C programs and the
benefits of this design approach.
15. State the meaning of the source code items that are underlined:
int out(int16 t)
{
int16 n;
while (input(PIN_D0))
{ outbyte++;
for(n=1;nt;n++);
}
return outbyte;
}
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16. Outline briefly the format of the RS232 signal and how it is used to operate a
serial alphanumeric LCD.
17. Draw a simple flowchart to represent a function to scan the keys of a numerical
keypad and return a code for a key press.
18. Explain the meaning of each component of the statement
printf(%d,anum[n]);
19. Explain the significance of the and * operators in C.
20. State the function of the compiler directives:
(a) #include.
(b) #define.
(c) #use.
(d) #device.
(e) #asm.
Assignments 2
To undertake these assignments, install Microchip MPLAB (www.microchip.com),
Labcenter ISIS Lite (www.proteuslite.com), and CCS C Lite (www.ccsinfo.com).
Application files may be downloaded from www.picmicros.org.uk. Run the applications in
MPLAB with Proteus VSM selected as the debug tool. Display the animated schematic in
VSM viewer, with the application COF file attached to the MCU (see the appendices for
details).
Assignment 2.1
Download the OUTBYTE.DSN file and attach ENDLESS.COF. Check that it works
correctly. Modify the program so that the LED output LSB flashes at 4Hz. Predict the
frequency of the MSB and measure it using the simulation clock.
Assignment 2.2
Download the SIREN project files and check that the SIREN program in Listing 2.7
works correctly. Modify the program to produce a default output at 1kHz. Further
modify the program so that the output frequency is halved each time the input button is
pressed.
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Assignment 2.3
Download the CALC project files and check that the CALC program works correctly.
Modify the program such that the ON/C key must the pressed to start the program
and pressing it again disables the program. Investigate the use of the string processing
functions to provide a more elegant implementation of the conversion of an input string of
numbers to decimal during the input phase. Outline how the program could be developed
to handle floating point numbers to provide a more practical calculator.
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C Peripheral Interfaces
3.1 PIC16 C Analog Input
● Analog input display
● Voltage measurement
● ADC setup codes
A microcontroller analog input allows an external voltage to be converted to digital
form, stored, and processed. This type of input occurs in data loggers, control systems,
digital audio, and signal processors, to mention just a few. The dsPIC range is designed
specifically for high-speed analog signal processing.
Analog Setup
A basic setup to demonstrate analog input is shown in Figure 3.1. The PIC16F877 has
eight analog inputs, which are accessed via RA0, RA1, RA2, RA3, RA5, RE0, RE1,
and RE2, being renamed AN0 to AN7 in this mode. All these pins default to analog
operation, but a combination of analog and digital inputs can be selected using the system
function set_up_adc_ports().
These inputs are multiplexed into a single converter, so they can be read only one at a
time. The function set_ADC_channel(n) selects the input channel. The analog-to-
digital converter module has a resolution of 10 bits, giving a binary output of 0x000 to
0x3FF (102310). Therefore, the measurement has a precision of 1/1024100%, which
is slightly better than 0.1%. This is good enough for most practical purposes. A 16-bit
integer or floating point variable is needed to receive this result.
P A R T 3
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Alternatively, the low-resolution mode can be used if an 8-bit conversion is sufficiently
precise (output 0–255). This mode is selected using the directive #device ADC=8.
The function read_ADC() then returns the input value as an unsigned integer. The
default input voltage range is 0–5V, which does not give an exact conversion factor. In
the demo program, Listing 3.1, the 8-bit input value is divided by 32 to give an arbitrary
voltage level from 0 to 8. This is then converted to the ASCII code by adding 0x30
and sending it to the display. The operation is repeated endlessly, using the statement
for(;;), which means execute a for loop unconditionally.
Voltage Measurement
The circuit shown in Figure 3.2 allows the input voltage at each analog input to be
displayed. An external reference voltage (2.56V) is connected to RA3, which sets the
maximum of the input range. This allows a more accurate and convenient scaling of the
measurement. The reference voltage is supplied by a zener diode and voltage divider
RB0/INT
13
RV1
1k
33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
14
1
2
3
4
5
6
7
8
9
10
LCD1
MILFORD-2X16-BKP
VDD
RXD
VSS
Figure 3.1: Single Analog Input and Display Test Circuit
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circuit. The value of the zener load resistor has been selected by simulation to adjust the
voltage to 2.560 0.1%. A potentiometer is connected to each of the measured inputs so it
can be set to an arbitrary test value. The test program VOLTS.C is provided in Listing 3.2.
This time, the ADC resolution is set to 10 bits, to obtain a more precise reading. Floating
point array variables are declared for the input readings (0–1023) and the calculated
voltage. The reference voltage, 2.56V, is represented by the maximum conversion value,
1024, so the scaling factor is 1024/2.56 400 bits per volt. The input is therefore divided
by this factor to obtain a display in volts. Note that, in the division operation, both values
must be float types.
The ADC port setup code selects all inputs as analog, with RA3 an external reference
(although this is not obvious from the select statement format). All the possible
Listing 3.1 Source Code for Simple Analog Input Test Program
/* ANALIN.C MPB 5-1-07
Read display analog input
***************************************************************/
#include 16F877A.h
#device ADC=8 //8-bit conversion
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1) //LCD output
void main() //*************************************************
{
int vin0; // Input variable
setup_adc(ADC_CLOCK_INTERNAL); // ADC clock
setup_adc_ports(ALL_ANALOG); // Input combination
set_adc_channel(0); // Select RA0
for(;;)
{ delay_ms(500);
vin0=read_adc(); //Get input byte
vin0=(vin0/32)+0x30; //Convert to ASCII
putc(254); putc(1); delay_ms(10); // Clear screen
printf(Input=); putc(vin0); // Display input
}
}
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combinations of analog and digital inputs are given in the 16F877A.H header file, Listing
2.19. When the program is compiled, the define statement selected is replaced by the
corresponding hex code, which is then loaded into the ADC control register to set up
the ADC.
The set of functions that control the ADC are listed in Table 3.1. The function
setup_adc() allows the clock rate (ADC sampling rate) to be selected to suit the
application, and setup_adc_ports() allows the mix of analog and digital inputs
to be defined using the combinations provided in the header file.
RB0/INT
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
14
1
2
3
4
5
6
7
8
9
10
RV5
RV4
RV3
R1
680R
R3
RV5(1)
V2.71739
LCD1
MILFORD-2X16-BKP
U1(RA3/AN3/VREF)
V2.55838
VDD
RXD
VSS
120R
RV6
RV7
RV8
RV1
RV2
R2
4k7
D1
1k
1k
1k
1k
1k
1k
1k
1k
BZX79C2V7
Figure 3.2: Input Voltage Measurement and Display
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Listing 3.2 Test Program for Voltage Measurement
/* VOLTS.C MPB 25-3-07
Read display 10-bit input voltage
*****************************************************************/
#include 16F877A.h
#device ADC=10 // 10-bit operation
#use delay(clock=4000000)
#use rs232(baud=9600,xmit=PIN_D0,rcv=PIN_D1)
void main() //**************************************************
{
int chan;
float analin[8], disvolts[8]; // Array variables
setup_adc(ADC_CLOCK_INTERNAL); // ADC Clock source
setup_adc_ports(AN0_AN1_AN2_AN4_AN5_AN6_AN7_VSS_VREF); // ADC inputs
while(1) // Loop always
{
for(chan=0;chan8;chan++) // Read 8 inputs
{ delay_ms(1000); // Wait 1 sec
set_adc_channel(chan); // Select channel
analin[chan]=read_adc(); // Get input
disvolts[chan]=(analin[chan])/400; // Scale input
putc(254);putc(1);delay_ms(10); // Clear display
printf( RA%d = %4.3g,chan,disvolts[chan]); // Display volts
}
}
}
Table 3.1: CCS C Analog Input Functions
Action Description Example
ADC SETUP Initialize ADC setup_adc(ADC_CLOCK_INTERNAL);
ADC PINS SETUP Initialize ADC pins setup_adc_ports(RA0_ANALOG);
ADC CHANNEL SELECT Select ADC input set_adc_channel(0);
ADC READ Read analog input inval=read_adc();
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3.2 PIC16 C Interrupts
● C interrupt functions
● Interrupt sources
● External interrupt
Interrupts allow an external event to initiate a control sequence that takes priority over
the current MCU activity. Typically, the interrupt service routine (ISR) carries out some
operation associated with the port or internal device that requested the interrupt.
Interrupts are frequently used with hardware timers, which provide delays, timed
intervals, and measurement. A time delay can be implemented using a simple software
counting loop, but this has the disadvantage of tying up the processor while the delay
executes. A more efficient technique is to use a hardware timer running independently
from the MCU clock. This allows accurate timing to be more easily achieved, and the
timer can run concurrently with some other task. A time-out interrupt informs the MCU
that the timer interval has expired and the ISR can implement the required action. The
interrupt has to be initialized for use at the top of the program.
C Interrupts
The CCS C complier provides a set of functions that implement the PIC interrupt system
(Table 3.2). The interrupt sources available in the PIC16F877 are as listed in Tables 3.3
Table 3.2: CCS C Interrupt Functions
Action Description Example
INTERRUPT CLEAR Clears peripheral interrupt clear_interrupt(int_timer0);
INTERRUPT
DISABLE
Disables peripheral interrupt disable_interrupts(int_timer0);
INTERRUPT
ENABLE
Enables peripheral interrupt enable_interrupts(int_timer0);
INTERRUPT
ACTIVE
Checks if interrupt flag set interrupt_active(int_timer0);
INTERRUPT EDGE Selects interrupt trigger edge ext_int_edge(H_TO_L);
INTERRUPT JUMP Jump to address of ISR jump_to_isr(isr_loc);
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Table 3.3: 16F877 Primary Interrupts
Interrupt Label Interrupt Source
GLOBAL Use to enable all interrupt sources
INT_EXT External interrupt detect on RB0
INT_RB Change on Port B detect
INT_RTCC Timer 0 overflow (same as TIMER0)
INT_TIMER0 Timer 0 overflow (same as RTCC)
and 3.4. These predefined labels must be used when enabling individual interrupts and
declaring the ISR block. They are defined in the header file along with the initialization
codes for the interrupt control registers.
Table 3.4: 16F877 Peripheral Interrupts
Interrupt Label Interrupt Source
Ports
INT_TBE USART transmit data done
INT_RDA USART receive data ready
INT_SSP Serial data received at SPI or I2C
INT_BUSCOL I2C collision detected
INT_PSP Data ready at parallel serial port
Timers
INT_TIMER1 Timer 1 overflow
INT_CCP1 Timer 1 capture or compare detect
INT_TIMER2 Timer 2 overflow
INT_CCP2 Timer 2 capture or compare detect
Others
INT_AD Analog-to-digital converter complete
INT_COMP Comparator output change
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Interrupt Example
Program INTEXT.C (Listing 3.3) demonstrates the basic interrupt setup. An output count
represents the primary task. This is interrupted by the switch input at RB0 going low,
forcing the execution of the interrupt service routine, which causes all the output LEDs to
come on for 1 second. The original task is then automatically resumed at the point where
it was interrupted. It is designed to run on the hardware shown in schematic Figure 3.3.
When the RB0 interrupt is detected during the main loop, the context (current register
contents) is saved before the ISR executed. If the program execution is studied carefully,
it can be seen that the original count prior to the interrupt is restored to the port output
after the interrupt. The ISR includes code to save and restore the MCU registers, so that
the main task can be resumed unaffected by the interrupt. Only local variables should be
used in the ISR to protect the integrity of the rest of the program.
Listing 3.3 External Interrupt Test Program Source Code
// INTEXT.C MPB 10-4-07
// Demo external interrupt RB0 low interrupts foreground output count
#include 16F877A.h
#use delay(clock=4000000)
#int_ext // Interrupt name
void isrext() // Interrupt service routine
{ output_D(255); // ISR action
delay_ms(1000);
}
void main() //********************************************
{
int x;
enable_interrupts(int_ext); // Enable named interrupt
enable_interrupts(global); // Enable all interrupts
ext_int_edge(H_TO_L); // Interrupt signal polarity
while(1) // Foreground loop
{
output_D(x); x++;
delay_ms(100);
}
}
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Interrupt Statements
The program statements associated with interrupt operation are as follows.
#int_ext
This directive tells the compiler that the code immediately following is the service
routine for this particular interrupt. The routine is the form in a standard function,
with a function name appropriate to the ISR task, in this case void isrext(). The
interrupt name is preceded by # (hash) to mark the start of the ISR definition and to
differentiate it from a standard function block. An interrupt name is defined for each
interrupt source.
RB0/INT
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
1
2
20
19
3
4
5
6
7
8
9
10
9
8
7
6
5
4
3
2
1
19
20
21
22
27
28
29
30
18
17
16
15
14
13
12
11
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
U1
R1
10k
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
14
1
2
X1
C2
C1
U2 RP1
220R
15pF
15pF
4MHz
3
4
5
6
7
8
9
10
Figure 3.3: External Interrupt Test Hardware
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enable_interrupts(int_ext);
This statement in the main program block enables the named interrupt by loading the
necessary codes into the interrupt control registers. These are defined in the device header
file by association with the interrupt label.
enable_interrupts(global);
This is required in all cases, allowing all interrupts to be enabled or disabled together.
The corresponding global disable function might be used to turn off all interrupts when a
timing critical task is to be executed.
ext_int_edge(H_TO_L);
The active edge of the external input can be selected as the falling (H_TO_L) or rising
(L_TO_H) edge. As in this example, a manual switched input is usually wired as active
low, and the falling edge is therefore used. On the other hand, it may be preferable to use
the rising edge, since there is no switch bounce when the contacts are opening.
Further examples of interrupts are provided later among the peripheral interfacing demo
programs.
3.3 PIC16 C Hardware Timers
● Counter/timers
● Capture and Compare
● Timer interrupt
The PIC 16F877 has three hardware timers built in: Timer0 (originally called RTCC, the
real-time counter clock), Timer1, and Timer2. The principal mode of operation of these
registers are as counters for external events or timers using the internal clock. Additional
registers are used to provide Capture, Compare, and Pulse Width Modulation (PWM)
modes. The CCS timer function set is shown in Table 3.5.
Counter/Timer Operation
A counter/timer register consists of a set of bistable stages (flip-flops) connected in
cascade (8, 16, or 32 bits). When used as a counter, a pulse train fed to its least significant
bit (LSB) causes the output of that stage to toggle at half the input frequency. This is fed
to the next significant bit, which toggles at half that rate, and so on. An 8-bit counter thus
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counts up from 0x00 to 0xFF (255) before rolling over to 0 again (overflow). The binary
count records the number of clock pulses input at the LSB.
In the ‘877, Timer0 is an 8-bit register that can count pulses at RA4; for this purpose,
the input is called T0CKI (Timer0 clock input). Timer1 is a 16-bit register that can count
up to 0xFFFF (65,535) connected to RC0 (T1CKI). The count can be recorded at any
chosen point in time; alternatively, an interrupt can be generated on overflow to notify the
processor that the maximum count has been exceeded. If the register is preloaded with a
suitable value, the interrupt occurs after a known count.
The counters are more frequently used as timers, with the input derived from the MCU clock
oscillator. Since the clock period is accurately known, the count represents an accurate timed
period. It can therefore be used to measure the period or frequency of an input signal or
internal intervals or generate a regular interrupt. Many PIC MCUs incorporate one or more
Capture, Compare, and PWM (CCP) modules that use the timer registers.
A timer/counter register may have a prescaler, which divides the input frequency by
a factor of 2, 4, 8, and so forth using additional stages, or a postscaler, which does
the same at the output. Timer0 has a prescaler that divides by up to 128; Timer1 has
one that divides by 2, 4, or 8; and Timer2 has a prescaler and postscaler that divide by
up to 16.
PWM Mode
In Pulse Width Modulation mode, a CCP module can be used to generate a timed output
signal. This provides an output pulse waveform with an adjustable high (mark) period.
Table 3.5: Timer Functions
Action Description Example
TIMERX SETUP Set up the timer mode setup_timer0(RTCC_
INTERNAL|RTCC_DIV_8);
TIMERX READ Read a timer register (8 or 16 bits) count0 get_timer0();
TIMERX WRITE Preload a timer register (8 or 16 bits) set_timer0(126);
CCPX SETUP Select PWM, capture, or compare
mode
setup_ccp1(ccp_pwm);
PWMX DUTY Set PWM duty cycle set_pwm1_duty(512);
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The high output state, called the duty cycle, is expressed as a percentage of the overall
period of the pulse wave. A duty cycle of 50% gives an equal mark and space ratio.
Program PWM.C (Listing 3.4) shows the basic setup procedure.
The setup_ccp1() function selects the mode of operation of the CCP module. The
function setup_timer_2()controls the overall period of the PWM wave and has three
arguments. The first sets the timer prescale division ratio, 16 in this case. The prescaler is
an additional counter stage that reduces the input clock rate by the selected ratio of 1, 4,
or 16. The second argument gives the overall output period from 1 to 255 times the input
clock period. The last value is the postscaler setting, from 1 to 16, which divides
the output from the MSB before it is fed to the interrupt system, so that the interrupt
period can be adjusted to be a multiple of the timer output. The duty cycle is set via the
set_pwm1_duty() function call. The value given is in the range 1–1023, an initial value
for a 10-bit counter. The value 500 gives a mark-space ratio of about 50%.
The PWM wave is generated continuously after the setup is completed. The values
for duty cycle (500) and overall period (248) used in this example produce an output
at CCP1 of 250Hz (4ms) and a mark-space ratio of 50% with a 4-MHz MCU clock.
The overall period is derived as follows: Timer2 is driven from the instruction clock at
1MHz (Fosc/4). After prescaling, the clock period is 16μs and the timer counts up to
248, overflowing approximately every 16248 3968 μs or about 4ms (the figure 248
is used rather that 250 to adjust for software overheads in the timer processing). The
postscaler value is set to default ‘1,’ since the timer interrupt is not being used in this
example.
Listing 3.4 Pulse Width Modulation Program Source Code
// PWM.C MPB 11-4-07
// Demo PWM output, MCU clock=4MHz
#include 16F877A.h
void main()
{
setup_ccp1(ccp_pwm); // Select timer and mode
set_pwm1_duty(500); // Set on time
setup_timer_2(T2_DIV_BY_16,248,1); // Clock rate output period
while(1){} // Wait until reset
}
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The various setup options available for the timers and CCP modules are given in the
16F877 header file in Listing 2.19. Refer to the CCS User Manual for more details about
using these options.
Compare Mode
PWM uses the compare operation illustrated in Figure 3.4 to generate a timed output
in conjunction with Timer2. The 16-bit CCPR register is preloaded with a set value,
which is continuously compared with the Timer1 count. When the count matches the
CCPR value, the output pin toggles and a CCP interrupt is generated. If this operation is
repeated, an interrupt and output change with a known period can be obtained.
Capture Mode
This mode uses the timer in the inverse manner to compare. The CCP pin is set to input
and monitored for a change of state. When a rising or falling edge (selectable) is detected,
the timer register is cleared to 0 and starts counting at the internal clock rate. When the
next active edge is detected at the input, the timer register value is copied to the CCP
register. The count therefore corresponds to the period of the input signal. With a 1-
MHz instruction clock, the count is in microseconds. An interrupt can also be generated
on each active edge. The general hardware configuration is shown in Figure 3.5, and a
program to demonstrate this operation is shown in Listing 3.5.
In the main block of Program PERIOD.C, Timer1 and the CCP mode are set up
(RE rising edge of signal to be captured). The required interrupt is enabled, and the
program waits for the CCP1 interrupt, indicating that the next rising edge has arrived.
The CCP1 interrupt service routine clears the timer and interrupt, ready for the next
Preload
Set Interrupt
Flag (CCP1lF)
Set/Clear
Pin RC2
Instruction
Clock
CCPR1L
CCPR1H
Comparator
TMR1H TMR1L
Figure 3.4: Compare Hardware Block Diagram
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capture event. The captured value is copied automatically into a variable called CCP_1.
The simulation of this program is shown in Figure 3.6. When the program is run with the
100-Hz signal input, a count of 9963μs is captured (error 0.4%). This shows that some
allowance may be needed for the software overhead associated with the capture process
and adjustment made to correct the result obtained.
Listing 3.5 Capture Mode Demo Program
// PERIOD.C MPB 11-4-07
// Demo of period measurement
#include 16F877A.h //****************************
#int_ccp1 // Interrupt name
void isr_ccp1() // Interrupt function
{
set_timer1(0); // Clear Timer1
clear_interrupt(INT_CCP1); // Clear interrupt flag
}
void main() //************************************
{
setup_timer_1(T1_INTERNAL); // Internal clock
setup_ccp1(CCP_CAPTURE_RE); // Capture rising edge on RC2
enable_interrupts(GLOBAL); // Enable all interrupts
enable_interrupts(INT_CCP1); // Enable CCP1 interrupt
while(1){}
}
Figure 3.5: Capture Hardware Block Diagram
Capture
Enable
Set Interrupt
Flag (CCP1lF)
Pulse Input
Pin RC2
Instruction
Clock
CCPR1L
CCPR1H
Prescale
Edge Select
TMR1H TMR1L
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3.4 PIC16 C UART Serial Link
● RS232 port functions
● Simulation with virtual terminal
A basic serial link is provided by the UART. We have already seen that any pair of pins
can be used for this interface, as the data rate is quite low, allowing the signals to be
generated in software. However, a dedicated hardware port is provided, which must be
used if an interrupt is needed. The CCS C library functions associated with this port are
listed in Table 3.6.
The UART can be tested in simulation mode by connecting it to the virtual terminal
provided in Proteus VSM, as shown in Figure 3.7. The terminal input RXD (receive data)
is connected to the PIC MCU TX (transmit) pin, and the TXD (transmit data) output is
connected to PIC MCU RX (receive). It has additional handshaking (transmission control)
lines RTS and CTS, but these are not usually needed.
Figure 3.6: Capture Mode Used to Measure Input Period
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The program listed as HARDRS232.C (Listing 3.6) is attached to the MCU in the
simulator. The getc() function is used to read a character from the virtual terminal; it
waits for user input. The terminal must be activated by clicking inside terminal window,
and the computer keyboard then provides the input to the PIC as the corresponding ASCII
codes; these are assigned to the variable incode, as they arrive.
The ASCII code can be output using printf(). If formatted as a decimal, the numerical
value of the character code is displayed. Alternatively, the character formatting code %c is
used to display the character itself. The function putc(13)outputs the code for a line return
on the display. If putc()is used to output an ASCII code, the character is displayed.
Table 3.6: RS232 Serial Port Functions
Title Description Example
RS232 SET BAUD RATE Set hardware RS232
port baud rate
setup_uart(19200);
RS232 SEND BYTE Write a character to the
default port
putc(65)
RS232 SEND SELECTED Write a character to
selected port
s=fputc(A,01);
RS232 PRINT SERIAL Write a mixed message printf(Answer:%4.3d,n);
RS232 PRINT SELECTED Write string to selected
serial port
fprintf(01,Message);
RS232 PRINT STRING Print a string and write
it to array
sprintf(astr,Ans=%d,n);
RS232 RECEIVE BYTE Read a character to an
integer
n=getc();
RS232 RECEIVE STRING Read an input string to
character array
gets(spoint);
RS232 RECEIVE
SELECTED
Read an input string to
character array
astring=fgets(spoint,01);
RS232 CHECK SERIAL Check for serial input
activity
s=kbhit();
RS232 PRINT ERROR Write programmed error
message
assert(a3);
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Listing 3.6 Hardware UART Demo Program
// HARDRS232.C MPB 13-6-07
// Serial I/O using hardware RS232 port
#include 16F877A.h
#use delay(clock=8000000) // Delay function needed for RS232
#use rs232(UART1) // Select hardware UART
void main() //************************************
{
int incode;
setup_uart(9600); // Set baud rate
while(1)
{ incode = getc(); // Read character from UART
printf( ASCII = %d ,incode); // Display it on
putc(13); // New line on display
}
}
Figure 3.7: RS232 Peripheral Simulation
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3.5 PIC16 C SPI Serial Bus
● SPI system connections
● SPI function set
● SPI test system
The serial peripheral interface master controller uses hardware slave selection to identify
a peripheral device with which it wishes to exchange data (refer to Section 1.4 for full
details of the signaling protocol). The available set of SPI driver functions are shown in
Table 3.7.
The test system has a slave transmitter that reads a binary-coded decimal input from a
thumbwheel switch and sends it to the master controller. This resends the code to the
slave receiver, which outputs to a BCD display (0–9). Each of three devices needs its own
test program to make the system work. The test system hardware is shown in Figure 3.8
and the individual test programs as Listings 3.7, 3.8, and 3.9.
As seen in the schematic, the slave MCUs are permanently enabled by connecting their
slave select inputs to ground. This is possible because there is only one sender on the
master input, so there is no potential contention. In a system with more that one slave
sender, each would need a separate slave select line, with only one being enabled at a time.
The individual programs were created as separate projects in MPLAB but saved in the
same folder, sharing a copy of the MCU header file. The COF files were then attached to
the corresponding chip in the simulated hardware.
Table 3.7: SPI Function Set
Operation Description Example
SPI SETUP Initializes SPI serial port setup_spi(spi_master);
SPI READ Receives data byte from SPI port inbyte=spi_read();
SPI WRITE Sends data byte via SPI port spi_write(outbyte);
SPI TRANSFER Sends and receives via SPI inbyte=spi_xfer(outbyte);
SPI RECEIVED Checks if SPI data received done=spi_data_is_in();
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RB0/INT
A1
A2
A3
A4
A5
A6
A7
A8
B1[0..7]
B2[0..7]
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877
PROGRAMspimaster.cof
CLOCK4MHz
CFGWORD0x3731
Master
SPIC.DSN
Demonstrates SPI read
from slave transmitter
and write to slave receiver
via master controller
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
14
1
2
3
4
5
6
7
8
9
10
RB0/INT
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
PIC16F877 PROGRAMspitransmit.cof
Slave Transmitter
PIC16F877 PROGRAMspireceive.cof
Slave Receiver
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/TOCKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
U2
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
SW1
20
21
22
27
BCD
28
29
30
13
14
1
2
3
4
5
6
7
8
9
10
RB0/INT
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
U3
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
13
14
1
2
3
4
5
6
7
8
9
10
Figure 3.8: SPI Test System Schematic
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Listing 3.7 SPI Slave Transmitter Source Code
// SPITRANSMIT.C MPB 20-6-07
// Serial I/O using SPI synchronous link
// Simulation hardware SPIC.DSN, transmitter program attached to U2
#include 16F877A.h
void main() //*****************************************
{
int sendnum;
setup_spi(spi_slave); // Set SPI slave mode
while(1)
{ sendnum = input_D(); // Get BCD input
spi_write(sendnum); // Send BCD code to master
}
}
Listing 3.8 SPI Master Controller Source Code
// SPIMASTER.C MPB 20-6-07
// Serial I/O using SPI synchronous link
// Simulation hardware SPIC.DSN, master program, attach to U1
#include 16F877A.h
void main() //***************************************************
{
int number;
setup_spi(spi_master); // Set SPI master mode
while(1)
{ number spi_read(); // Read SPI input BCD code
spi_write(number); // Resend BCD code to slave
}
}
3.6 PIC16 C I2
C Serial Bus
● I2
C simulation test system
● I2
C control, address, and data bytes
The inter-integrated circuit (I2
C) synchronous serial bus provides a means of
exchanging data between peripheral devices and microcontrollers using software
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Listing 3.9 SPI Slave Receiver Source Code
// SPIRECEIVE.C MPB 20-6-07
// Serial I/O using SPI synchronous link
// Simulation hardware SPI.DSN, receiver program, attach to U3
#include 16F877A.h
void main() //***************************************
{
int recnum;
setup_spi(spi_slave); // Set SPI slave mode
while(1)
{ recnum=spi_read(); // Read BCD code at SPI port
output_D(recnum); // Display it
}
}
addressing. This means that only two signals are required, data and clock (see Section 1.4
for details).
The test system shown in Figure 3.9 has only one I2
C peripheral device, the 24AA256
serial flash memory chip, to keep it as simple as possible. Serial memory is a common
feature of applications that require additional data storage, such as a data logger. It allows
the internal EEPROM of the PIC to be expanded using only two I/O pins. The downside
is that the memory access is rather slow, with the maximum write cycle time of 5ms (200
bytes/sec) specified for this device. Therefore, the data sampling rate needs to be suitably
modest.
The serial memory chip has a capacity of 256-k bits, or 32-k bytes, with three external
address pins: A0, A1, and A2. This allows a set of up to eight chips to be used in the
system, each with a different hardware address, 0–7. This address is included in the
address code sent by the master controller, so that a specific address in a selected chip can
be accessed. With eight 32-k chips, the total address space is 256k. In the test system, the
memory chip hardware address is 000.
The system reads a test code set manually on Port B inputs, which is copied to the
serial memory. Pull-ups must be fitted to the serial clock and data lines, and a virtual
I2
C analyzer is also attached to the bus. The test program writes the test byte (3F in the
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128. 120 Part 3
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example shown) to the address lowadd, which increments from 0 after each write. The
i2c_start() function initiates the data transfer sequence, by generating a start bit on
the data line. This is followed by 4 bytes, containing control, address, and data codes.
The first is the control code, A0. The memory chip has a factory-set high address code of
0101(A). This distinguishes it from other types of I2
C devices that may be added to the
bus. The next 3 bits are the hardware address (000), and the LSB is set to 0 to indicate a
write operation, making the low nibble 0000. This is followed by the two address bytes.
The high address byte is 00, and the low address increments from 0, so the test program
writes only to the first 256 bytes. The data byte follows, which is read in from the input
switches.
Each of these bytes must be acknowledged by the receiving device taking the data line
low, and the transfer is terminated by a stop bit. More details on the exact data format and
timing requirements may be found in the chip data sheet.
The simulation system allows the bus activity to be logged and displayed in the I2
C debug
window using the virtual bus monitor instrument. A time stamp, the transfer codes, and
Figure 3.9: I2
C Test System
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129. C Peripheral Interfaces 121
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the Start (S), Acknowledge (A), and Stop (P) bits are detected as they occur. In addition,
the memory contents can be displayed to confirm the test data and which locations have
been written.
When the memory content window is opened, we see that it retains the data from
previous runs of the simulation, representing the nonvolatile nature of the data store. To
see the data change, a new code must be set on the switches for each run.
The I2
C functions are summarized in Table 3.8.
3.7 PIC16 C Parallel and Serial Interfaces
● PSP functions and test system
● Comparison of parallel and serial links
The parallel slave port (PSP) allows an external controller to initiate an 8-bit data
exchange with the PIC MCU. This method of data exchange is compared with the serial
ports.
Parallel Slave Port
In the example in Figure 3.10, a master ‘877 is feeding data to a slave chip of the same
type. Arbitrary data are set on the DIP switch at Port B of the master. The internal
pull-ups available on these pins are activated in the master program to avoid the need for
external resistors on the switches. The test data are transferred to Port C and presented to
the slave Port C pins (Listing 3.10). The slave port is already enabled via E0 (!CS not
Table 3.8: I2
C Functions
Operation Description Example
I2C WRITE Send a single byte i2c_write(outbyte);
I2C READ Read a received byte inbyte=i2c_read();
I2C STOP Issue a stop command in master mode i2c_stop();
I2C POLL Check to see if byte received sbit=i2c_ poll();
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RB0/INT
13 33
34
35
36
37
38
39
40 20
19
18
17
16
15
14
13
12
11
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
RN1
270R
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
19
20
21
22
27
28
29
30
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RB0/INT
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
PlC16F877 PlC16F877
U1 MASTER U2 SLAVE
33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
16
15
14
13
12
11
10
9
1 9
2
3
4
5
6
7
8
14
1
2
3
4
5
6
7
8
9
10
13
14
1
2
3
4
5
6
7
8
9
10
RA0/AN0
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
RA0/AN0
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
ON OFF
Figure 3.10: PSP Test System
Listing 3.10 PSP Master Test Program
// PSPMASTER.C
// Test system master controller program, design file PSP.DSN, U1
#include 16F877A.h
void main() //**************************************
{
int sendbyte;
port_b_pullups(1); // Activate Port B pull-ups
while(1)
{ sendbyte = input_B(); // Get test byte
output_D(sendbyte); // Output on PSP bus
output_low(PIN_E2); // Select PSP slave
output_low(PIN_E1); // Write byte to slave port
output_high(PIN_E1); // Reset write enable
}
}
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131. C Peripheral Interfaces 123
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chip select) on Port E, and the data are latched in when E1 (!WR not write) is pulsed
low by the master.
In simulation mode, the write pulse frequency was measured at 40kHz (MCU clock
4MHz). The slave program (Listing 3.11) monitors the receive flag associated with the
port and picks up the data when the port indicates that data have been loaded into the PSP
data register. The data then are transferred to Port C for display on the bar graph.
A parallel external bus can thus be created that connects microcontrollers, extra memory,
and other 8-bit devices to form a system similar to a conventional microprocessor system.
On the PSP bus, the master must select the peripheral device to be accessed using the chip
select mechanism. If necessary, an address decoding system can be added to expand the
hardware without using extra master pins. For example, a 3-bit decoder generates eight
chip select signals. A memory space is created for the master, where different peripherals
are accessed at separate address ranges.
Table 3.9 summarizes the PSP functions.
Comparison of Communication Links
We can now compare the available PIC MCU communication ports so that the most
suitable can be selected for any given application. Table 3.10 summarizes the main features.
Listing 3.11 PSP Slave Test Program
// PSPSLAVE.C
// Test system slave controller program, design file PSP.DSN, U2
#include 16F877A.h
void main() //****************************************
{
int recbyte;
setup_psp(PSP_ENABLED); // Enable PSP slave port
while(1)
{ if(psp_input_full()) // If data have been received
{ recbyte=input_D(); // Copy in test data
output_C(recbyte); // Display data on bar graph
}
}
}
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As we have seen, three serial communication interfaces are available plus the parallel
slave port.
In theory, the parallel port should be the fastest, because 8 bits can be transferred at a
time. The PSP can be used to create a multiprocessor system with a common data bus
connected to same port on other MCUs, with one master controlling the addressing
system and selecting the slave MCU. One example of such a multiprocessor system is a
robot with a separate controller for each motor. The master controller sends data to the
motor slaves to set position, speed, or acceleration of that axis. Data transfer speed may
be crucial to optimum system performance, so the parallel connection may be preferred in
this case. This is feasible as long as the physical distance between the controller and the
motors is not too far.
For serial data transfer, speed (bits per second) increases as we progress from UART
through I2
C to SPI. As well as being the fastest, SPI is also relatively simple to implement.
It can operate in Multimaster mode but needs hardware slave selection. I2
C needs only
two wires and operates like a mini-network, so it may be more effective for larger systems.
However, the software is more complex and carries a significant addressing overhead.
The UART is a simple way to link a single master and slave and allows greater link
distance by use of line drivers. On the other hand, it does not support any form of
multiprocessor or bus system.
Table 3.9: PSP Functions
Operation Description Example
PSP SETUP Enables or disables PSP setup_psp(PSP_ENABLED);
PSP DIRECTION Sets the PSP data direction set_tris_e(0);
PSP OUTPUT READY Checks if output byte is ready
to go
pspo = psp_output_full();
PSP INPUT READY Checks if input byte is ready to
read
pspi = psp_input_full();
PSP OVERFLOW Checks for data overwrite error pspv = psp_overflow();
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Table 3.10: Comparison of PIC Communication Ports
UART SPI I2
C PSP
Description Serial RS232, Host–
terminal, single link
Serial data, bus
connection with
hardware selection
Serial data and address,
bus connection with
software addressing
Parallel 8-bits, bus
connection with
hardware control
Clock Asynchronous Synchronous, max
5MHz
Synchronous, max
5MHz
Synchronous
Wiring TX, RX, GND SCK, SDI, SS SCL, SDA 10-k
pull-ups
PSP0–PSP7, RD,
WR, CS
Data 6–9 bits 8 bits serial 8 bits address
control
Page mode option
8 bits parallel
Control Start, Stop bits Clock strobe Clock strobe, Start,
Acknowledge
Read, Write, Chip
Select
Speed
(bits/sec)
LOW 19.2kb/sec HIGH 5Mb/sec HIGH 1–5Mb/sec,
depends on mode
MID 40
8240kb/sec1
Distance2
HIGH 100m LOW 1m LOW 1m LOW 1m
Nodes 2 only Unlimited3
1024 (10-bit address) Limited by bus
characteristics
Systems Single peer to peer Master/slave Master/slave Master/slave
Operation Can be connected
as a simple 2-
wire system but
has additional
handshaking modes
and parity checking
for extra reliability
Simple clocked
data, high speed
but requires slave
selection wiring and
possibly external
decoding
Complex software
control and addressing
reduces speed but
requires no slave
selection wiring or
external decoding
hardware
Simple hardware
control but with
limited bus length.
Higher speeds
possible using
assembler routine.
May need external
decoding.
Typical
applications
PC host to MCU
target data transfer
(e.g., data logger)
Sensor data
link, MCU
communication link
Multiperipheral control
system with sensors and
low-speed memory data
storage
Multiprocessor
system, parallel
MCU data link
Notes:
1
This is an estimated speed using nonoptimized C code to drive the bus. If optimized assembler code were
used, this could be improved significantly.
2
Transmission distance in the UART is enhanced by using line drivers to increase the signal voltage to
overcome line impedance and interference. Data transmission at TTL signal levels in the other links restricts
the distance to within the same subsystem (board, unit, or back plane). For greater distances and multinode
operation, a local area network interface is required, which provides synchronous data communication with
unlimited software addressing and error correction.
3
The SPI system can be expanded by additional address decoding and line drivers as necessary, but there are
practical limits to this option, and I2
C or networking would probably be more effective.
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3.8 PIC16 C EEPROM Interface
● EEPROM test system
● EEPROM test program
The internal electrically erasable programmable read only memory block is not strictly
speaking a peripheral, as it is internal to the MCU, but it is accessed in a way similar to
external devices so it is included in this part. In the 16F877, the EEPROM is a block of
256 bytes of nonvolatile read/write memory. It allows data to be retained while the power
is off, which is useful in applications such as an electronic lock where a secure code
needs to be stored.
Figure 3.11 shows a test circuit that demonstrates its operation. Arbitrary 8-bit codes
are set on the switch bank, which are stored, recalled, and displayed on the LED bank.
The R/!W (Read/Not Write) input switch is closed to select the Write mode. The switch
code is set and the button pressed. This stores the code in the first EEPROM location,
RB0/INT
13 33 ON OFF
34
35
36
37
38
39
40
1 9
2
3
4
5
6
7
8
R1
10k
C1
1nF
R2
10k
R/!W
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
16
15
14
13
12
11
10
9
3
2
1
4
5
6
7
8
9
10
18
19
20
Step
17
16
15
14
13
12
11
1
2
3
4
5
6
7
8
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF/CVREF
RA3/AN3/VREF
RA4/T0CKI/C1OUT
RA5/AN4/SS/C2OUT
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877A
U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
14
2
3
4
5
6
7
8
9
10
1
Figure 3.11: EEPROM Test System
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address 0. The switch code is then changed and the next code stored in location 1, and so
on until a 0 is entered on the switches. As the data are stored, each byte is displayed on the
bar graph.
The R/!W switch is then opened to select read mode. As the button is pressed, the same
sequence of stored codes is displayed from memory. The nonvolatile data storage is
demonstrated by the fact that the test data are retained between successive simulation
runs. This can be viewed if the simulation is paused and the EEPROM data window
selected from the debug menu. Listing 3.12 is an EEPROM test program.
3.9 PIC16 C Analog Output
● Waveform generator test system
● Waveform test program
● Waveform output
In microcontroller applications, analog output is not needed as often as analog input,
so no digital to analog converter (DAC) is built into the PIC MCU. An external DAC is
needed to generate analog output signals.
A serial DAC may be used to output a precision DC reference voltage or low-frequency
analog signal, using SPI or I2
C to transfer the data. A 10-bit or 12-bit output is typically
provided, giving a precision of about 0.1 or 0.025%, respectively. However, the serial data
transfer is inherently slow. In the demo system described here (Figure 3.12), higher speed
is possible with parallel output to the DAC. The waveform generator circuit generates
trigonometric waveforms, which are displayed on the virtual digital oscilloscope.
The system provides 8-bit conversion, giving a precision of 100/2560.4%. With a
20-MHz MCU clock, the maximum output frequency is about 4kHz. This is limited by
the maximum rate at which the output loop can produce the instantaneous voltages that
make up the waveform.
The DAC code is output at Port D, with a variable delay to control the frequency. A set
of switches provides waveform selection and push-button frequency adjustment. The
DAC0808 produces a current output that needs an external amplifier to convert it to a
voltage and provide the output drive. The amplifier stage also allows the output amplitude
and offset to be adjusted.
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Listing 3.12 EEPROM Test Program
// EEPROM.C
// Internal data EEPROM test, design file EEPROM.DSN
#include 16F877A.h
#use delay(clock=4000000)
void main() //////////////////////////////////////////////////////////
{
int writebyte, readbyte;
int maxadd, address;
port_b_pullups(1); // Enable Port B internal pull-ups
if(!input(PIN_C1)) // Write memory sequence //////////////////
{
address=0; // First address
do
{ while(input(PIN_C0)){}; // Wait for button
writebyte = input_B(); // Get switch bank data
write_eeprom(address,writebyte); // Write data to EEPROM
readbyte = read_eeprom(address); // Read it back
output_D(readbyte); // Display data on bar graph
while(!input(PIN_C0)){}; // Wait for button release
address++; // Next EEPROM address
} while(writebyte!=0); // Continue until data = 00
}
else // Read memory sequence ///////////////////
{
address = 0; // First address
do
{ while(input(PIN_C0)){}; // Wait for button
readbyte = read_eeprom(address); // Read data
output_D(readbyte); // Display it on bar graph
while(!input(PIN_C0)){}; // Wait for button release
address++; // Next address
} while(readbyte!=0); // Continue until data = 00
while(1); // Done *************************************
}
}
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RB0/INT SQUARE
SINE
TRIANG
ARBIT
WAVEFORM
PERIOD UP
PERIOD DOWN
RESTART
NOWAVE
OUTPUT AMPLITUDE
RV2 22k
U3
D
4
2
3
6
7
1
5
C
B
A
13 33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
19
20
21
22
27
28
29
30
12
11
10
9
8
7
6
5
16
3
4
15 10k
10k
RV1
R1
R4
7k5
10k
15V
R3
OUTPUT
OFFSET
DAC0808
R2 15k
15V
15V
TL071
100nF
C1
14
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
PlC16F877A
U2
1 5
2
3
4
14
1
2
3
4
5
6
7
8
9
10
1
RA0/AN0
RA1/AN1
RA2/AN2/VREF
RA3/AN3/VREF
RA4/TOCKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
MCLR/Vpp/THV
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
ON OFF
A8
U1
A7
A6
A5
A4
A3
A2
A1
VEE
COMP
IOUT
VREF
VREF
50%
Figure 3.12: Waveform Generator
The program source code is shown in Listing 3.13. This is only a demonstration of the
digital waveform generator principle, and a more sophisticated design is required to produce
a waveform with a better resolution at higher frequencies. It serves only to illustrate some
relevant features of C and the principle of waveform synthesis that may be used in high-
performance digital signal processors, such as the dsPIC range. This is an application where
critical sections of the code could be written in assembler for higher speed.
The main object of the program is to generate instantaneous voltages in sequence to
produce a square, sine, triangular, and arbitrary waveform. The mid-value for the output
is 10010. Instant values ranging between 100 and 100 are added to this value to
produce the output.
For the arbitrary pattern, most values are 0 in this example, with an increasing value at
intervals of ten steps. This produces a pulse-modulated triangular waveform, which might
be used to test a digital filter, but any other repetitive pattern can be entered as required.
The arbitrary sequence is generated from the values entered into the array amp[n] in
the function setwave() at the source code edit stage. A mechanism for entering these
externally in hardware could easily be added, but that is rather tedious to demonstrate.
For the other waveforms, the values are calculated. The square wave is just a set of
constant maximum (100) and minimum (100) values, and the triangular wave is an
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Listing 3.13 Waveform Generator Source Code
// DACWAVE.C MPB 5-7-07
// Outputs waveforms to DAC, simulation file DAC.DSN
#include 16F877A.H
#include MATH.H
#use delay(clock=20000000)
#use fast_io(D) // High speed output functions
int n, time=10;
float step, sinangle;
float stepangle = 0.0174533; // 1 degree in radians
int amp[91]; // Output instant voltage array
// ISR to read push buttons ******************************************
#int_rb
void change()
{
if(time!=255)
{if (!input(PIN_B4)) time ++;} // Increase period
while(!input(PIN_B4));
if(time!=0)
{if (!input(PIN_B5)) time--;} // Decrease period
while(!input(PIN_B5));
if(!input(PIN_B6))reset_cpu(); // Restart program
if(!input(PIN_B7))for(n=0;n91;n++)amp[n]=0; // Zero output
}
void setwave() // Arbitrary waveform values **********************
{
amp[0] =00;amp[1] =00;amp[2] =00;amp[3] =00;amp[4] =00;
amp[5] =00;amp[6] =00;amp[7] =00;amp[8] =00;amp[9] =00;
amp[10]=10;amp[11]=00;amp[12]=00;amp[13]=00;amp[14]=00;
amp[15]=00;amp[16]=00;amp[17]=00;amp[18]=00;amp[19]=00;
amp[20]=20;amp[21]=00;amp[22]=00;amp[23]=00;amp[24]=00;
amp[25]=00;amp[26]=00;amp[27]=00;amp[28]=00;amp[29]=00;
amp[30]=30;amp[31]=00;amp[32]=00;amp[33]=00;amp[34]=00;
amp[35]=00;amp[36]=00;amp[37]=00;amp[38]=00;amp[39]=00;
amp[40]=40;amp[41]=00;amp[42]=00;amp[43]=00;amp[44]=00;
amp[45]=00;amp[46]=00;amp[47]=00;amp[48]=00;amp[49]=00;
amp[50]=50;amp[51]=00;amp[52]=00;amp[53]=00;amp[54]=00;
amp[55]=00;amp[56]=00;amp[57]=00;amp[58]=00;amp[59]=00;
amp[60]=60;amp[61]=00;amp[62]=00;amp[63]=00;amp[64]=00;
amp[65]=00;amp[66]=00;amp[67]=00;amp[68]=00;amp[69]=00;
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amp[70]=70;amp[71]=00;amp[72]=00;amp[73]=00;amp[74]=00;
amp[75]=00;amp[76]=00;amp[77]=00;amp[78]=00;amp[79]=00;
amp[80]=80;amp[81]=00;amp[82]=00;amp[83]=00;amp[84]=00;
amp[85]=00;amp[86]=00;amp[87]=00;amp[88]=00;amp[89]=00;
amp[90]=90;
}
void main() //*************************************************
{
enable_interrupts(int_rb); // Port B interrupt for buttons
enable_interrupts(global);
ext_int_edge(H_TO_L);
port_b_pullups(1);
set_tris_D(0);
// Calculate waveform values ***********************************
step=0;
for(n=0;n91;n++)
{
if(!input(PIN_B0)) amp[n] = 100; // Square wave offset
if(!input(PIN_B1)) // Calculate sine values
{ sinangle = sin(step*stepangle);
amp[n] = floor(sinangle*100);
step = step+1;
}
if(!input(PIN_B2)) amp[n] = n; // Triangular wave
if(!input(PIN_B3)) setwave(); // Arbitrary wave
}
// Output waveform vales ***************************************
while(1)
{ for(n=0;n91;n++) {output_D(100+amp[n]); delay_us(time);}
for(n=89;n0;n--) {output_D(100+amp[n]); delay_us(time);}
for(n=0;n91;n++) {output_D(100-amp[n]); delay_us(time);}
for(n=89;n0;n--) {output_D(100-amp[n]); delay_us(time);}
}
}
incrementing and decrementing count. The sine output is the most interesting, as it is
calculated using the sine function from the math.h library. These values are assigned to
the amp[n] array for output after being calculated, since to calculate each and output it
“on the fly” would be too slow.
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The waveform is selected at the start of the program by polling the selection switch bank.
If the waveform selection is changed, the loop must be restarted using the push button.
On the other hand, the frequency may be modified while the output is running. The main
consideration here is the timing of the output waveform—each step must take the same
time. The minimum step time is also important, as this determines the highest frequency.
Therefore, input polling is avoided. Instead, the Port B change interrupt is used to detect
the push buttons, and the period modification and waveform control operations are
placed in the interrupt routine void change(). Here, the delay between each output
step is incremented or decremented or the loop stopped and restarted. The sine waveform
obtained is illustrated in Figure 3.13.
Assessment 3
5 points each, total 100
1. Write a C statement that sets up the PIC ADC so that only RA0 is used as an
analog input. Deduce the resolution per bit for a 10-bit conversion, assuming a 5V
supply.
2. If a single 4.096V reference voltage is connected to Vref and 10-bit conversion
completed, write a C statement (a) to declare suitable variables and (b) to
convert the input value to the actual voltage for display.
Figure 3.13: Sine Wave DAC Output
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3. List the statements required to set up an ADC interrupt and outline the related
ISR initialization if it is called “isrADC.”
4. Explain the advantages of using an interrupt to read the data from an analog
input conversion, compared with simply checking it on a regular basis (polling)
within the program loop.
5. A 16-bit timer is preloaded with a value of 15,536. The MCU clock runs at 8MHz,
with a prescaler set to divide by 16. Calculate the timer output interval obtained.
6. Explain briefly the difference between the Capture and Compare modes of
operation.
7. Draw a labeled diagram to show a PWM waveform, indicating how the
overall period and duty cycle are set by the arguments of functions setup_
timer_2(a,b,c) and set_PWMx_duty(d). The MCU instruction clock
period is T.
8. Calculate modified parameters for the setup functions in program PWM that
produce an output at 1kHz with a duty cycle of 10% (0.1-ms pulse). The
instruction clock is 1MHz.
9. Explain why the UART is a suitable interface for transmission of characters to a
serial LCD display, especially if the LCD is separated from the MCU board.
10. Explain the effect of the statements printf(%d,incode) and
putc(incode) on an LCD display connected to an MCU serial output, if the
value of incode is 0x41.
11. Outline how to structure a program using interrupts that can carry on some other
task while the serial data are transferred to and from the UART, and explain why
this might be useful.
12. By reference to Section 1.4, explain briefly how the hardware and master
program would be modified if more than one slave sender were in the SPI
system shown in Figure 3.8.
13. List the sequence of I2
C statements to write the data byte 0xAA to address
0x01FF in the serial memory chip in the system shown in Figure 3.9.
14. Draw a block diagram showing how to connect two PIC MCUs using an I2
C
link.
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15. Describe the sequence of operations required to write a byte to the parallel
slave port of the PIC MCU and to force the slave MCU to read the data in
immediately.
16. By reference to Table 3.10, select a serial link that connects numerous PIC
MCUs to a master controller using the minimum number of wires, and explain
briefly why this not the fastest method to read from a serial peripheral.
17. Select from Table 3.10 the most suitable communication link for each of these
applications, one for each method:
(a) An interface to a conventional memory chip with 8-bit data access.
(b) A robot control system with one master MCU and six motor control
slaves.
(c) An MCU data logger uploading to a PC spreadsheet.
(d) A multiprocessor system with shared serial memory and sensors.
18. Describe briefly the function of EEPROM and its applications. Why is external
EEPROM sometimes necessary?
19. Explain why interrupts are used in the demo program DACWAVE to respond to
manual input to change the output frequency.
20. Outline how a simple program could produce a high-speed square wave using
the hardware in Figure 3.12.
Assignments 3
To undertake these assignments, install Microchip MPLAB (www.microchip.com), Labcenter
ISIS Lite (www.proteuslite.com), and CCS C Lite (www.ccsinfo.com). Application files may
be downloaded from www.picmicros.org.uk. Run the applications in MPLAB with Proteus
VSM selected as the debug tool. Display the animated schematic in VSM viewer, with the
application COF file attached to the MCU (see the appendices for details).
Assignment 3.1
Download the project ANALIN, the 8-bit analog test project. Run it and check that the
output voltage is represented by a number between 0 and 8. Now, modify the program to
display the actual voltage, bearing in mind that the reference value is 5V. That is, when
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the input is a maximum of 5V, the value received by the ADC will be 256. The scaling
factor therefore is 5/256 19.5mV/bit. The input therefore needs to be multiplied by
0.0195 to be displayed as voltage. Floating point variables need to be used in the revised
program.
Assignment 3.2
Download the project PWM and test it for correct operation. A 250-Hz (4-ms) pulse
waveform with a 50% duty cycle should be observed on the display. Now rewrite the
program to produce the same output using compare mode in Timer1. The timer needs
to run for 2ms for each half cycle; assuming a 4-MHz MCU clock and a 1-MHz timer
clock, a compare value of 2000 is needed.
Assignment 3.3
Download the project DACWAVE and test it for correct operation. Measure the minimum
and maximum frequencies available. Modify the arbitrary waveform data to produce a
step waveform that has amplitude 0 for five steps of the output, 5 for the next five steps,
10 for the next five steps, and so on until the amplitude reaches 90 over the last five steps,
then reduces to 0 again. It should then produce the same over the negative half cycle of
the waveform before repeating.
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C Mechatronics Applications
4.1 PICDEM Mechatronics Board Overview
● Mechatronics board hardware
● Mechatronics board connections
● Mechatronics board motor drives
The PICDEM mechatronics demonstration board (Figure 4.1), supplied by Microchip®
Inc., is a very useful target system for C control applications. A user manual, which can be
downloaded from www.microchip.com, contains the schematics and general guidance on
using the board. It can be programmed using the ICD2 In-Circuit Debugger module, which
allows a final stage of fault finding when testing an application in the target hardware.
Alternatively, the low-cost PicKit2 programmer can be used. Since our applications here
have been tested in simulation mode, the full ICD debugging interface is not needed.
PICDEM Hardware
The block diagram, Figure 4.2, shows the main parts of the mechatronics board. It is built
around a PIC 16F917, which is similar to the 16F877A but incorporates an LCD driver
module, which allows a plain 3.5-digit display to be operated with no additional interfacing.
The MCU internal clock runs at 8 MHz, giving a 0.5-s instruction cycle. The main
output devices are a small DC motor and a stepper motor. These are operated from a set
of four current driver FETs, which can sink or source current. These allow either motor
to be driven in both directions when connected as a full bridge driver. Input tactile push
switches and output LEDs are provided for simple test programs, mode selection, and
P A R T 4
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Figure 4.1: PICDEM Mechatronics Board (by permission of Microchip Inc.)
status indication. An RS232 serial port for exchanging data with the PC host is fitted,
which requires a suitable terminal program running on the PC.
A temperature sensor is fitted, which outputs 10 mV/C with 0C giving 500 mV.
Therefore, at 20°C, the output will be 500 (20 10) 700 mV. This voltage can be
fed to an ADC input or comparator input on the MCU. A light sensor is also available,
giving an output in the range 0–5 V. Two pots, giving 0–5 V, can be used as reference
inputs for the analog sensors or as test inputs for analog applications.
The mechatronics board has its main signals brought to in-line connectors, as shown in the
board layout (Figure 4.3). The components can be connected up for different applications
using link wires. The connector pin functions are listed for reference in Tables 4.1 through 4.4.
Motor Drives
The motors are driven from a set of four half-bridge driver stages, which can handle up to
1 A each. These can be connected to the 5-V regulated or the 9–12-V unregulated supply
for higher power output. Note that the main plug supply may be rated at less than 1 A, so
a separate supply is advisable if the full drive current is needed.
Each driver has a pair of MOSFETs, which allow the stage to source or sink current,
depending on which transistor is switched on (Figure 4.4). Control logic prevents both
coming on at the same time and shuts down all the drives if an overcurrent fault is detected.
This is activated on power up for fail safe operation and must be cleared manually before
testing a motor. If the DC motor needs to be driven in both directions, the half-bridge
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PIC 16F917 MCU
8 MHz
POT1
POT2
Push
Switch
2,3,4
Reset SW1
In-Circuit
Serial
Programming
Temp
Sensor
Light
Sensor
32.768 kHZ
XTAL Clock
RS 232
Serial Link
3.5-Digit
Display
x8
x4
LEDs
• • • • • • • •
Available I/O
Available I/O
J14
!MCLR
ICSPDAT
ICSPCLK
RA0/AN0
RA1/AN1
RA3/AN3
RA4
RA5/AN4
Drive Supply
5V or 12V
Half
Bridge
Driver
1,2,3,4
Source Enable Px
Control PWMx
Sink Enable Nx
Motor
Winding
0 – 1A
P9
P10
P11
P12
P9 P10
J7
P9
P10
P11
P12
Step
Motor
Optical
Interrupter
2 pulses/rev
Current
Sense
J15
Fault
(Shutdown)
Fault
(Shutdown)
Motor
Winding
Currents
Current
Sense
Circuit
Clear
Fault Curent
Sense
0 – 1V
0 – 5V
J4 pins 3,4
0 – 5V
J4 pin 2
10mV/°C
0°C
500mV
J4 pin 1
Active Low
J4 pins
6,7,8
DC
Motor
OSC1
OSC2
RX
TX
LCD
RD2/CC
P2
Figure 4.2: Block Diagram of PICDEM Mechatronics Board
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Figure 4.3: Mechatronics Board Layout (by permission of Microchip Inc.)
stages are connected as shown. Pairs of FETs are switched on to allow the current to flow
diagonally through FET1 and FET4 or FET3 and FET2, reversing the current in the load.
An additional control input allows PWM control of the drives. This involves switching
the current on and off over a set period and varying the average current by changing the
mark-space ratio. The PIC has two CCP modules that use the internal hardware timers to
provide the required output at CCP1 and CCP2 (see Part 3).
The DC motor needs some form of feedback if it is to be controlled accurately. It
therefore has a slotted wheel attached to its output shaft, which passes between an LED
and opto-sensor. The sensor produces a pulse for each slot, two per revolution, which
allows the motor position and speed to be measured by the MCU. Alternatively, provision
is made for speed measurement using back emf, where the drive is switched off for a
short period in the cycle and the voltage generated by the motor measured. The back emf
is proportional to the speed while the motor is working as a tachogenerator.
The stepper motor has two sets of windings, which are activated in sequence. This moves
the rotor one step at a time, or 7.5 degrees. The windings are connected to separate full-
bridge drivers consisting of half-bridges 1/2 and 3/4.
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Table 4.1: Mechatronics Board Fixed Connections
Label Alt Func MCU Pin Function
Dedicated I/O
SW1/!MCLR RE3 1 Reset MCU (if enabled in fuses)
ICSPDATA RB7 40 In-circuit serial programming data
ICSPCLK RB6 39 In-circuit serial programming clock
RX RC7 26 Receive data from RS232 interface
TX RC6 25 Transmit data to RS232 interface
Display I/O
SEG0 RB0 33 LCD segment 0 (see display map)
SEG1 RB1 34 LCD segment 1 (see display map)
SEG2 RB2 35 LCD segment 2 (see display map)
SEG3 RB3 36 LCD segment 3 (see display map)
SEG6 RC3 18 LCD segment 6 (see display map)
SEG21 RE0 8 LCD segment 21 (see display map)
SEG22 RE1 9 LCD segment 22 (see display map)
SEG23 RE2 10 LCD segment 23 (see display map)
COM0 RB4 37 LCD Common connection 0
COM1 RB5 38 LCD Common connection 1
COM2 RA2 4 LCD Common connection 2
COM3 RD0 19 LCD Common connection 3
VLCD1 RC0 15 LCD control voltage 1 (Vdd/3 1.66 V)
VLCD2 RC1 16 LCD control voltage 2 (2 Vdd/3 3.33 V)
VLCD3 RC3 17 LCD control voltage 3 Vdd
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Table 4.2: Mechatronics Board User Connections
User input devices
SW2 General purpose tactile switches (active low),
SW3 use RA0, RA1, RA3, RA4, RA5, RA6, RA7
SW4
POT1 Manual analog input (0–5V) for ADC and comparator,
POT2 use AN1 (C1), AN2, AN3 (C1), AN4
Sensor inputs
TEMP Temperature sensor (10mV/°C, 0°C 500 mV), use AN1–AN4
LIGHT Light sensor (0–5 V), use C1 and C1
Table 4.3: DC Motor Connections
Label Alt Func MCU Pin
DC motor output (J1)
P1 RD7 Enable source current driver stage 1
PWM1 CCP1 Pulse width control driver stage 1
N1 RD6 Enable sink current driver stage 1
P2 RD5 Enable source current driver stage 2
PWM2 CCP2 Pulse width control driver stage 2
N2 RD4 Enable sink current driver stage 2
DC motor sensors
OPTINT J7 Optical interrupter, 2 pulses per rev, use CCP1
BACKEMF J16 Back EMF, 0–5 V, use RA1
CSENSE J15 Current measurement, 1 mV/mA, use RA1
All bridge drives are connected to ground via a 0.1- current sensing resistor, which
produces a voltage proportional to the load current. This is fed to an amplifier and
comparator so that the current can be measured. The comparator triggers a “fault”
condition if the current exceeds 1 A (100 mV across the sensing resistor), which shuts
down the drives. This fault condition also occurs on power-up, ensuring that the drives
start only after the Clear Fault switch is pressed.
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Table 4.4: Stepper Motor Connections
Label Alt Func MCU Pin
P1 RD7 Enable source current driver stage 1
PWM1 CCP1 Pulse width control driver stage 1
N1 RD6 Enable sink current driver stage 1
P2 RD7 Enable source current driver stage 2
PWM2 CCP1 Pulse width control driver stage 2
N2 RD6 Enable sink current driver stage 2
P3 RD5 Enable source current driver stage 3
PWM3 CCP2 Pulse width control driver stage 3
N3 RD5 Enable sink current driver stage 3
P4 RD5 Enable source current driver stage 4
PWM4 CCP2 Pulse width control driver stage 4
N4 RD4 Enable sink current driver stage 4
Test Program
An initial test program for the PICDEM board is used to check that the downloading and
in-circuit debugging modes are operational. The system setup is shown in Figure 4.5,
the test program outline in Listing 4.1, and the source code in Listing 4.2. The program
outline can be used in more complex applications to help to construct the program.
Figure 4.4: Full-Bridge Driver Connection of the DC Motor
FET
1
FET
2
FET
3
FET
4
Motor
Reverse
0 V
Vs
Forward
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Listing 4.1 Test Program Outline
TEST
Include 16F917 header file
Use delay library routines
Count=0
Loop always
Output count at Port D
Delay 10ms
Increment count
Figure 4.5: Block Diagram of Test Hardware Configuration
ICD2
Download
Debug
Program
PICDEM1
RD7
RD6
RD5
RD4
D7
LEDS
D6
D5
D4
PIC
16F917
Listing 4.2 Mechatronics Board Test Program
//TEST.C MPB 14-4-07
//First program for testing Mechatronics Board
//Flashes 4 LEDs, total cycle time = 256 × 10ms = 2.56s
//Connect RD7-D7, RD6-D6, RD5-D5, RD4-D4
#include 16F917.h // Device header file
#use delay(clock=8000000) // Delay function clock speed
void main() //Start main block
{
int n=0; //Count loop variable
while(1) //Endless loop
{
output_D(n); //Show on LEDs
delay_ms(10); //Wait 10ms between steps
n++; // Increment loop count
}
} //End of source code
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A program implements a simple output loop, which increments the binary count at
Port C. The PIC 16F917 outputs RD4 to RD7 need to be connected to the LEDs D3 to D7
on the target board with link leads on the connector pins. The ICD2 module is plugged
into the board via the ICD connector and to a host PC USB port.
The source code is loaded or edited in the usual way within MPLAB and saved in a
project folder called “test.” The source code and device header file are placed in the project
folder and attached to the project in the project file window. Assuming the C compiler has
been previously installed, the project can be complied and the HEX and COF files
created.
The program is downloaded by selecting the menu Programmer, Select Programmer,
MPLAB ICD2. Confirmation that the target is ready should appear in the output window.
Hit the Program Target Device button and ideally a Programming Succeeded message is
returned. The Release from Reset button should set the output running on the LEDs on
the mechatronics board.
Debugging
If a program does not function correctly, it can be debugged in hardware using ICD2.
For this exercise, we run the program in debug mode anyway. From the Debugger menu,
Select Tool MPLAB ICD2. If necessary, the operating system in the ICD module is
updated. A reminder may be received that the ICD2 module cannot operate as a debugger
and programmer at the same time. An error message may be displayed at this stage,
indicating that the system cannot enter debug mode. Resend the program and try again.
The output window should then show that the target system is ready.
The debug control panel now appears in the toolbar, allowing the program to Run,
Stop, Reset, or Single Step. The current execution point is displayed in the source
code window. Reset the program if necessary, and run it. The LEDs should flash in a
binary sequence on the target board. Stop the program and set a breakpoint at the output
statement in the source code. Open the watch window and display the value of ‘n’ in
binary. It increments each time the loop is executed, but note that the output shows only
the most significant 4 bits. It therefore changes only after a count of 16.
You will find that the step-over function does not work. This is probably because the
subroutine calls in CCS C are implemented using the assembler instruction GOTO
instead of CALL, which the step-over function is expecting. This can be confirmed by
opening the Disassembly Listing in the View menu.
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The debug windows are shown in Figure 4.6. When debugging is complete, clear all
breakpoints and ensure that the program is working as required. After the final version
is downloaded and the ICD module disconnected, the program should run from Reset on
power-up.
4.2 PICDEM Liquid Crystal Display
● LCD layout and connections
● LCD test program
● BCD count program
The plain 3.5-digit parallel liquid crystal display (LCD) is driven directly from the MCU,
occupying 15 of the I/O pins. The usual alternative to this arrangement is to use a serial
LCD, which can be driven via the RS232 port. This occupies only one or two pins, but it
is more expensive, as it contains its own microcontroller.
LCD Connections
The parallel LCD is operated by specific combinations of inputs that enable the segments
as required (Figure 4.7). The segments are designated A to G for each seven-segment
Figure 4.6: Test Program Debugging Screen
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digit, with digits numbered 1 to 4 from the right. The most significant half digit (4) has
only segments B and C, displaying only ‘1’. Four common connections (COM1–COM4)
enable groups of segments such that each has a unique address.
Note that this is a standard DMM display, so additional symbols are available that are
not needed in the mechatronics board applications. The data for the display segments are
stored in dedicated set of 12 registers in the PIC 16F917 (Table 4.5), called LCDDATAx,
Figure 4.7: (a) LCD Segment Connections (courtesy of Varitronix Ltd.); (b) Segment
Labels; (c) MCU to LCD Connection; (d) LCD Connection Map
RC
PIN1
(a)
PIN14
DH
RH
AC
m VA
MK Ω
COMØ
LCD1
(c)
COM1
COM2
COM3
SEG1
SEG2
SEG3
SEG11
SEG6
SEG21
SEG22
SEG23
SEG16
SEGØ
1
2
3
4
5
6
7
8
9
10
11
12
13
14
COM1
COM2
COM3
COM4
RC/BATT/–/AC
DH/RH/B–C/4DP
3A/3F/3E/3D
3B/3G/3C/3DP
2A/2F/2E/2D
2B/2G/2C/2DP
1A/1F/1E/1D
1B/1G/1C/
S1/S2/m/M
A/V/K/omega
PIN
(d)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
COM1
RC
DH
3A
3B
2A
2B
1A
1B
S1
A
COM2
BATT
RH
3F
3G
2F
2G
1F
1G
S2
V
COM3
MINUS
4B,C
3E
3C
2E
2C
1E
1C
m
K
COM4
AC
DP3
3D
DP2
2D
DP1
1D
M
S3
COM1 COM2 COM3 COM4
A
(b)
G
B
F
D
C
E
P
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where ‘x’ is 0 to 11 (SFR addresses 110h–11Bh, bank 1). These registers contain 12
8 96 bits, which are identified individually, bits 0–95. If one of these bits is high, the
corresponding LCD segment or pixel is on.
The LCD has a total of 26 numerical segments, comprising three seven-segment digits,
two segments for the MSD, and three decimal points. The MSD bits are controlled by
the same bit, as they always come on together, giving only 25 bits actually required.
Therefore, only some bits in the registers are used, but the spare capacity allows more
complex displays to be operated by the ’917 in other applications. We see that the bits
that are used are not arranged very logically, so they will be mapped by the LCD display
function to simplify the output process.
The bits in the first three registers (LCDDATA0–LCDDATA2) are associated with COM0
output, the next three with COM1, and so on to COM3 (see Table 4.5). Unfortunately, the
Table 4.5: PIC 16F917 LCD RAM Data Register Bits
Address COM0+ Address COM1+
00+ -- 06 -- -- 03 -- -- -- 24+ -- 06 -- -- 03 -- -- --
LCDDATA0 xx 2A xx xx 3A xx xx xx LCDDATA3 xx 2F xx xx 3F xx xx xx
08+ -- -- -- -- 11 -- -- -- 32+ -- -- -- -- 11 -- -- --
LCDDATA1 xx xx xx xx 3B xx xx xx LCDDATA4 xx xx xx xx 3G xx xx xx
016+ 23 22 21 -- -- -- -- -- 40+ 23 22 21 -- -- -- -- --
LCDDATA2 1B 1A 2B XX XX XX XX XX LCDDATA5 1G 1F 2G xx xx xx xx xx
Address COM2+ Address COM3+
48+ -- 06 -- -- 03 02 -- -- 72+ -- 06 -- -- 03 02 -- --
LCDDATA6 xx 2E xx xx 3E 4x xx xx LCDDATA9 xx 2D xx xx 3D P3 xx xx
56+ -- -- -- -- 11 -- -- -- 80+ -- -- -- -- 11 -- -- --
LCDDATA7 xx xx xx xx 3C xx xx xx LCDDATA10 xx xx xx xx P2 xx xx xx
64+ 23 22 21 -- -- -- -- -- 88+ -- 22 21 -- -- -- -- --
LCDDATA8 1C 1E 2C xx xx xx xx xx LCDDATA11 xx 1D P1 xx xx xx xx xx
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common inputs on the LCD are identified as COM1–COM4, so COM1 is controlled from
the MCU output COM0, and so on, with COM4 being connected to COM3 MCU output pin.
The 16F917 MCU can provide up to 24 segment drive outputs (SEG0–SEG23), with four
common connections (COM0–COM3). These are used in defined combinations to control
up to 24 4 96 segments or pixels in the display. In this way, 1 bit in the LCDDATAx
registers controls one element of the display. This display needs only 25 bits and ten of
the available segment outputs (SEG0, 1, 2, 3, 6, 11, 16, 21, 22, and 23). These outputs are
encoded to allow individual bit control within the program.
LCD Test Program
Listing 4.3 shows a test program, LCD1, which displays the numerals 0 to 9 on each digit
in turn, then flashes on the MSD and three decimal points, so that correct operation of
each can be checked.
Listing 4.3 Test Program for Mechatronics Board LCD
// LCD1.C MPB 20-4-07
// Test program for mechatronics board LCD
// Displays count 0 to 9 on Digits1,2,3 and 1 on Digit4
#include 16F917.h
#use delay(clock=8000000)
//LCD DISPLAY DATA: (3 numerals *
7 segments)+ MSD *
1 segment + 3 decimal
points
//Bit map for numerals 0–9 and blank..................................
//Numeral: 0 1 2 3 4 5 6 7 8 9 blank
byte const DigMap[11]={0xFD,0x60,0xDB,0xF3,0x66,0xB7,0xBF,0xE0,0xFF,0xE7,0x00};
//Bit addressess in LCD RAM locations LCDDATA0 to LCDDATA11=12*
8 bits
//Numbered 0-95 with offsets COM0=0, COM1=24, COM2=48, COM3=72
//Segment: A B C D E F G
#define DIG1 COM0+22,COM0+23,COM2+23,COM3+22,COM2+22,COM1+22,COM1+23
//Bit addresses
#define DIG2 COM0+6, COM0+21,COM2+21,COM3+6, COM2+6, COM1+6, COM1+21
//Bit addresses
#define DIG3 COM0+3, COM0+11,COM2+11,COM3+3, COM2+3, COM1+3, COM1+11
//Bit addresses
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#define DIG4 COM2+2 //Both bits
#define DP1 COM3+21 //Decimal point 1
#define DP2 COM3+11 //Decimal point 2
#define DP3 COM3+2 //Decimal point 3
void main()
{
int8 n;
setup_lcd(LCD_MUX14,0); //Initialize 14-pin LCD, no clock
divide
for(n=0;n11;n++) //Display numerals 0–9 at digit 1
{ lcd_symbol(DigMap[n],DIG1); //Send digit bits to segment
addresses
delay_ms(300);
}
for(n=0;n11;n++) //Display numerals 0–9 at digit 2
{ lcd_symbol(DigMap[n],DIG2); //Send digit bits to segment
addresses
delay_ms(300);
}
for(n=0;n11;n++) //Display numerals 0–9 at digit 3
{ lcd_symbol(DigMap[n],DIG3); //Send digit bits to segment
addresses
delay_ms(300);
}
lcd_symbol(0X80,DIG4); //Switch on MSD digit 4
delay_ms(1000);
lcd_symbol(0X00,DIG4); //Switch off MSD digit 4
lcd_symbol(0XFF,DP1); //Switch on decimal point 1
delay_ms(500);
lcd_symbol(0X00,DP1); //Switch off decimal point 1
lcd_symbol(0XFF,DP2); //Switch on decimal point 2
delay_ms(500);
lcd_symbol(0X00,DP2); //Switch off decimal point 2
lcd_symbol(0XFF,DP3); //Switch on decimal point 3
delay_ms(500);
lcd_symbol(0X00,DP3); //Switch off decimal point 3
while(1){}; //Done
}
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Each group of segments associated with each common connection on the LCD is
operated in turn by the program. The LCD functions used are setup_lcd() and
lcd_symbol()
. The arguments of the setup function specify a 14-pin display module
and 0 clock divide factor. The clock rate controls the display multiplexing rate, which
can be modified for best visibility.
The arguments of the output function comprise an 8-bit map for the numeral to be
displayed as a hex number (Table 4.6) and a list of the corresponding bits in the
LCDDATAx locations for that digit. The 8-bit numeral codes are shown in Figure 4.7.
Because of the interaction of the control lines, the LSB for each code was determined by
inspecting the results on the display. Otherwise, the mapping is as normally required for
seven segment codes.
The mapping data for each segment is provided to the output function in the form of a
list of segment bit addresses, 0–95. To include information about which COM line is
active for each bit, the address is supplied as the sum of the start address of each COM
block and the bit number within that block. Therefore, the bit address of segment A of
digit 1 (DIG1) is COM0 22. COM0 has the value 0, COM1 24, COM2 48, and
COM3 72. Therefore, COM0 22 22. By the same process, the single-bit address
Table 4.6: Bit Maps for LCD Numerals
Numeral Segment A B C D E F G LSB Code
Bit 7 6 5 4 3 2 1 0
0 1 1 1 1 1 1 0 1 0xFD
1 0 1 1 0 0 0 0 0 0x60
2 1 1 0 1 1 0 1 0 0xDB
3 1 1 1 1 0 0 1 1 0xF3
4 0 1 1 0 0 1 1 0 0x66
5 1 0 1 0 0 1 1 1 0xB7
6 1 0 1 0 1 1 1 1 0xBF
7 1 1 1 0 0 0 0 0 0xE0
8 1 1 1 1 1 1 1 1 0xFF
9 1 1 1 0 0 1 1 1 0xE7
— 0 0 0 0 0 0 0 0 Blank
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controlling the MSD (DIG4) is COM2 2 50, and the first decimal point (DP1) is
addressed at COM3 21 93.
For convenience, the lists of segment bit addresses for each digit are defined at the top
of the program, using the replacement text labels DIG1, DIG2, DIG3, and DIG4 plus the
three decimal point addresses. The lcd_symbol() function is then supplied with the
constant array element number for the numeral to be displayed and the bit address list
as DIGx. A ‘for’ loop outputs each numeral at each position in turn, including the blank
digit, while the MSD and decimal points are switched on and off individually.
BCD Count Program
Listing 4.4 shows a program that displays a decimal count on the LCD. The count is
generated as binary coded decimal (BCDx) digits. Each digit is initialized to 0, then
incremented until it reaches 10, when it is cleared back to 0 and the next most significant
digit incremented. The three digits are then displayed together. The MSD (DIG4) is not
used. The LCD data block is now concealed in a separate source code file lcd.inc,
which is included at the top of the program.
4.3 PICDEM DC Motor Test Programs
● Motor test program
● Rev counter program
The primary target device on the board is the DC motor. The hardware configuration is
shown in Figure 4.8. The first program just switches the motor on and off, and the second
shows how to control the speed.
Basic Control
The minimal program (Listing 4.5) shows how to run the mechatronics board under the
control of SW2. The motor is connected to Drive1 and Drive2 output terminals, with
two output bits of the MCU linked to P1 and N2. When these go high, the motor current
is switched on in a forward direction. The output code 0x90 100100002 switches on
RD4 and RD7 when the switch input RA4 goes low. If desired, the PIC output pins can
also be monitored on the LEDs. The project should be loaded and tested as described in
Section 4.1.
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Listing 4.4 LCD Counting Program
/////////////////////////////////////////////////////////////////////
//LCD2.C MPB 20-4-07
//LCD program to count up when SW2 on
//Hardware: Connect SW2 to RA4
/////////////////////////////////////////////////////////////////////
#include 16F917.h
#include lcd.inc //Include file with LCD data
#use delay(clock=8000000)
void main() //////////////////////////////////////////////////
{
int8 BCD1=0, BCD2=0, BCD3=0; //BCD count digits
setup_lcd(LCD_MUX14,0); //Initialize 14-pin LCD
while(1)
{ //GENERATE DECIMAL COUNT
if(!input(PIN_A4)) //Test Switch 2
{
delay_ms(10); //Debounce and slow
BCD1++; //Increment ones
if(BCD1==10) //..up to 9
{
BCD1=0; //Reset ones
BCD2++; //Increment tens
if(BCD2==10) //..up to 90
{
BCD2=0; //Reset tens
BCD3++; //Increment hundreds
if(BCD3==10) //..up to 900
BCD3=0; //All reset to zero
}
}
}
//DISPLAY BCD DIGITS
lcd_symbol(DigMap[BCD1],DIG1); //Display Digit 1
lcd_symbol(DigMap[BCD2],DIG2); //Display Digit 2
lcd_symbol(DigMap[BCD3],DIG3); //Display Digit 3
} //Loop always
}/////////////////////////////////////////////////////////////END
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Listing 4.5 Motor Test Program
//MOTOR1.C MPB 17-4-07 PICDEM board test program
//Control motor from switch. Connect SW2-RA4, RD7-P1, RD4-N2
#include16F917.h
void main()
{
while(1)
{
if(!input(PIN_A4)) //Test switch
output_D(0x90); //Switch on motor
else output_D(0x00); //Switch off motor
}
}
Rev Counter
The system is now developed to measure the number of revolutions completed during a
short run. The motor is still attached to Drive1 and Drive2 outputs, but in addition, the
output from the opto-sensor (OPTO), which produces two pulses per rev, is connected
to the Timer1 input on the MCU (RC5/T1CLKI). The motor is switched on by pressing
SW2, and the number of revs is displayed when it is released. The maximum rev count
is 999 (1998 pulses), which takes about 20 sec to reach, assuming the motor is running
at about 3000 rpm. The program source code is given in Listing 4.6 and is outlined in
Listing 4.7.
Figure 4.8: Block Diagram of Motor Test System
Drive 1
Opto-Sensor
Run
Switch
LCD
M
PIC
16F917
Current
Source
Drive 2
Current
Sink
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Listing 4.6 Program to Display Motor Revs
/////////////////////////////////////////////////////////////////////
// MOTREVS.C
// Program to count motor revs
// PICDEM hardware: Connect SW2-RA4, RD4-N2, RD7-P1
/////////////////////////////////////////////////////////////////////
#include 16F917.h
#include lcd.inc //Include file with LCD data
#use delay(clock=8000000)
void main() /////////////////////////////////////////////////////////
{
int8 BCD1=0, BCD2=0, BCD3=0; //Initialize 3 digits
int8 huns=0, tens=0, ones=0; //and digit values
int16 count=0; //Receives timer count
setup_lcd(LCD_MUX14,0); //Initialize 14-pin LCD
setup_timer_1(T1_EXTERNAL); //Initialize rev counter
while(1) //Main loop start
{
while(input(PIN_A4)){}; //Wait for switch 2 on
delay_ms(10); //Debounce switch
//COUNT MOTOR REVSX2///////////////////////////////////////
set_timer1(0); //Reset counter
output_D(0x90); //Start motor
while(!input(PIN_A4)) //Wait while switch on
{ delay_ms(10); } //Debounce switch
output_D(0x00); //Motor off
count=get_timer1(); //Read counter
count=count/2; //2 pulses per rev
//CONVERT COUNT TO BCD/////////////////////////////////////
huns=tens=ones=0; //Reset digit values
while (count99) //Calculate hundreds
{ count=count-100; huns++; } //digit by subtraction
while (count9) //Calculate tens
{ count=count-10; tens++; } //digit by subtraction
ones=count;
//DISPLAY BCD DIGITS///////////////////////////////////////
lcd_symbol(DigMap[ones],DIG1); //Display Digit 1
lcd_symbol(DigMap[tens],DIG2); //Display Digit 2
lcd_symbol(DigMap[huns],DIG3); //Display Digit 3
} //Loop always
} /////////////////////////////////////////////////////////END
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Listing 4.7 Outline of Rev Counter Program
MOTREVS
Specify MCU 16F917
Include LCD function file
Initialize display digits to zero
Setup LCD
Setup timer as external pulse counter
Main loop
Display 3 digits on LCD
Wait for input switch on
Reset counter and start motor
Wait for input switch off
Stop motor
Convert timer count to 3 digit BCD
The timer is set up for external input using setup_timer_1(T1_EXTERNAL), and the
resulting count is read using get_timer1()
. The binary number obtained from the timer
is divided by 2 and converted to BCD by a process of successive subtraction, which is
simple if not elegant. The calculated digits are then displayed as in previous examples,
using the function lcd_symbol() to output the display digits and the include file
LCD.INC for the display encoding.
4.4 PICDEM Stepper Motor Control
● Stepper motor operation
● Stepper motor test program
● Speed and direction control
The main advantage of the stepper motor is that it provides position control without the
feedback required by a DC motor. It has stator windings distributed around a cylindrical
rotor, which has permanent or induced magnetic poles. The windings operate in groups to
move the rotor by a fraction of a revolution at a time (Figure 4.8).
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N N N
S
S
Figure 4.9: Bipolar Permanent Magnet Stepper Motor with Two Winding sets
Construction
The small stepper motor on the mechatronics board is an inexpensive permanent magnet
(PM) type, giving 7.5 degrees per step, 48 steps per revolution. It can also be moved in
half steps by suitable operation of the windings or even smaller steps (microstepping) by
suitable modulation of the winding current. The motor has two bipolar windings, which
means the current is reversed to change the polarity of the stator pole. The coils energize
two rings of poles, creating alternating north and south poles, which interact with the
permanent rotor poles (Figure 4.9).
Representative windings are shown Figure 4.10; in the actual motor, coils are distributed
around the whole circumference, multiplying the torque produced. Their terminals are
Figure 4.10: Stepper Motor Test System Connections
PIC
16F917
SW2 RD7 P1
RD6 P2
CCP1
Brown
Orange
Red
Yellow
Winding
Rotor
PWM1
Direction
Motor
control
logic
linked for
full bridge
drive
operation
RD5 P3
RD4 P4
CCP2 PWM3
SW3
Faster
SW3
Slower
Drive 1
Drive 2
Drive 3
Drive 4
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connected to the four driver outputs on the board, which are normally connected for full-
bridge operation. This allows the current to be reversed in the stator windings, reversing
the polarity of the stator poles. The stator coils are brought out to four color-coded wires,
which are connected to the driver terminals.
In more expensive motors, a smaller step (typically 1.8º) can be obtained with four sets of
windings. These motors usually have six wires, with a common connection for each pair
of windings.
Stepper Motor Test
The stepper motor is connected to the driver outputs, in clockwise order. The six driver
input links must be closed to enable full-bridge operation, since the bipolar motor
requires winding current in both directions. P1, P2, P3, and P4 inputs are connected to
RD4, RD5, RD6, and RD7, respectively. When run, the program generates the required
switching sequence on the coils to energize them in the right order. SW2/RA1 changes
the direction, and SW3/RA3 and SW4/RA4 allow the step speed to be varied.
Source code STEPTEST.C is shown in Listing 4.8. Only the control inputs P1, P2, P3,
and P4 need to be connected to outputs RD7–RD4 at this stage. Note that the stepper
motor terminal connections are not in numerical color order. As can be seen, no special
program setup is needed. The program simply switches on the drivers in the order 1,4,2,3
by outputting a suitable hex code to Port D. The delay is set so that the steps can be
counted visually. It is helpful to attach an indicator flag to the motor shaft, so that the
stepping can be seen more easily. The number of full steps per rev can then be
confirmed (48).
Program STEPSPEED, Listings 4.9 and 4.10, is a development of the basic program to
test the motor response to a range of step rates. The input tactile switches change the
speed by modifying the delay time parameter, which is set to 16 ms by default. This gives
speed of
16 ms/step 16 48 0.768 sec/rev 0.768 60 46 rpm
Direction Control
The stepper motor program can now be further developed to include direction control, as
shown in STEPDIR.C (Listing 4.11). The program has been restructured to incorporate
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Listing 4.8 Stepper Motor Test Program
// STEPTEST.C
// Test program for PICDEM Mechatronics Board stepper motor,
// basic full step mode. Connect RD7-P1, RD6-P2, RD5-P3, RD4-P4
// plus all 6 jumpers for full bridge mode
// Motor moves 48 steps per rev (7.5 deg/step)
///////////////////////////////////////////////////
#include 16F917.h
#use delay(clock=8000000)
void main()
{
while(1) //Loop always
{
output_D(0x80); //Switch on Drive 1
delay_ms(200);
output_D(0x10); //Switch on Drive 4
delay_ms(200);
output_D(0x40); //Switch on Drive 2
delay_ms(200);
output_D(0x20); //Switch on Drive 3
delay_ms(200);
}
}
Listing 4.9 Outline of Stepper Motor Speed Control Program
STEPSPEED
Specify MCU 16F917
Set default step delay time
Main loop
If Direction switch pulsed, Call Forward
If Direction switch pulsed, Call Reverse
Forward
Call Speed
Output one forward cycle (4 steps) to motor
Reverse
Call Speed
Output one reverse cycle (4 steps) to motor
Speed
If Up button on, halve step delay
If Down button on, double step delay
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Listing 4.10 Stepper Motor Speed Control Program
///////////////////////////////////////////////////////////////////////
// STEPSPEED.CMPB 22-4-07
// Program for PICDEM Mechatronics Board stepper motor, full step mode
// Connect RD7-P1, RD6-P2, RD5-P3, RD4-P4 plus all 6 jumpers for full
// bridge mode plus SW3-RA3 and SW4-RA4. Motor speed SW3 up SW4 down
///////////////////////////////////////////////////////////////////////
#include 16F917.h
#use delay(clock=8000000)
void main()
{
int8 time=16; // Variable step delay
while(1) //Loop always
{
//CHECK SWITCHES
if(!input(PIN_A3)) //Poll SW3
{ delay_ms(10); //Debounce
if(time!=1)time=time/2; //Not if min
}
while(!input(PIN_A3)){}; //Wait switch
if(!input(PIN_A4)) //Poll SW3
{ delay_ms(10); //Debounce
if(time!=128)time=time*2; //Not if max
}
while(!input(PIN_A4)){}; //Wait switch
//4 STEPS CLOCKWISE
output_D(0x20); delay_ms(time); //Step 1
output_D(0x40); delay_ms(time); //Step 2
output_D(0x10); delay_ms(time); //Step 3
output_D(0x80); delay_ms(time); //Step 4
}
}
a procedure for modifying speed. In the main loop, the reversing button is tested; by
default the motor runs forward and is reversed each time the button is pressed. Before each
sequence of four steps, the speed buttons are polled and the delay modified if requested.
The structure makes it easier to write the program with the right logical sequence. A flaw
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Listing 4.11 Stepper Motor Speed and Direction Control
///////////////////////////////////////////////////////////////////////
// STEPDIR.C PICDEM Mechatronics Board stepper motor speed and dirc.
// Connect RD7-P1, RD6-P2, RD5-P3, RD4-P4 plus all 6 jumpers(full bridge)
// SW2-RA2, SW3-RA3, SW4-RA4. Motor speed SW3 up SW4 down, motor dirc SW2
///////////////////////////////////////////////////////////////////////
#include 16F917.h //MCU select
#use delay(clock=8000000) //Internal clock
int8 time=16; //Default speed
//PROCEDURES////////////////////////////////////////////////////
void speed() //Halve or double speed //////////
{
if(!input(PIN_A3)) //Poll SW3
{ delay_ms(10); //Debounce
if(time!=1)time=time/2; //Not if min
}
while(!input(PIN_A3)){}; //Wait switch
if(!input(PIN_A4)) //Poll SW3
{ delay_ms(10); //Debounce
if(time!=128)time=time*2; //Not if max
}
while(!input(PIN_A4)){}; //Wait switch
}
void forward() //4 steps clockwise /////////////
{
speed();
output_D(0x20); delay_ms(time); //Step 1
output_D(0x40); delay_ms(time); //Step 2
output_D(0x10); delay_ms(time); //Step 3
output_D(0x80); delay_ms(time); //Step 4
}
void reverse() //4 steps counter-clockwise /////
{
speed();
output_D(0x80); delay_ms(time); //Step 4
output_D(0x10); delay_ms(time); //Step 3
output_D(0x40); delay_ms(time); //Step 2
output_D(0x20); delay_ms(time); //Step 1
}
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void main() //Main loop///////////////////////////////////////////
{
while(1) //Loop always
{
while(input(PIN_A2)) { forward(); } //Run forward
delay_ms(10); //Debounce
while(!input(PIN_A2)){}; //Wait until released
while(input(PIN_A2)) { reverse(); } //Run reverse
delay_ms(10); //Debounce
while(!input(PIN_A2)){}; //Wait until released
}
}
in the algorithm is that the program checks the buttons only after four steps, so the direction
and speed do not change immediately if the motor is running at low speed. This type of
problem can be solved using interrupts.
4.5 PICDEM Analog Sensors
● Light switch application
● Temperature display application
The mechatronics board is fitted with a light and temperature sensor, each of which
produces an analog output in the range of 0–5 V. In common with many sensors now
available, a signal conditioning amplifier is built in, so that no additional components are
needed to interface with an MCU.
Light Sensor
The light sensor can be tested using the analog comparator inputs of the 16F917, which
allow two input voltages to be compared. An output bit in a status register is set if the
positive input (C) is at a higher voltage than the negative input (C) or a reference
voltage. A range of setup options are defined in the header file.
The block diagram in Figure 4.11 shows the hardware configuration for this test. The
connector pin LIGHT, the light sensor output, is connected to RA0 (comparator input C)
and POT1 to RA3 (comparator input C), with LED D7 is assigned to RD7 to display the
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Listing 4.12 Outline of Light Sensor Test Program
LIGHTCON
Select MCU 16F917
Initialize comparator input
Main loop
If lightset level, switch output OFF
Else switch output ON
comparator state. When the light level is reduced, the output switches on. Conversely, it goes
off as the light is increased through the switching level, which is adjustable using POT1. This
simulates the operation of an automatic streetlight switch or security lamp. The program
LIGHTCON is outlined in Listing 4.12 and the source code shown in Listing 4.13.
As we see, in the program, only the setup function is needed, which assigns the comparator
inputs to Port A pins. Two comparators are available, and the setup used here is the same
for all comparator applications using this hardware. C1OUT is the bit label assigned to the
Comparator 1 output bit, which is tested using the if statement. The LED output is then
switched accordingly. The pot sets the switching level, and a desk lamp or flashlight was
found to work as a light source. The LED should go on when the light source goes off.
Temperature Measurement
The temperature sensor on the PICDEM board has an output of 10 mV/ºC, with 500 mV
0ºC (Figure 4.12). For this application, the TEMP pin, to which the temperature sensor
output is connected, is linked to the first analog input RA0 (AN0). When run, the
Figure 4.11: Comparator Test Setup
POT1
0–5 V LED
D0
RD7
C
C
MCU
Light
Sensor
0–5 V
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temperature is converted and displayed. The program TEMPDIS outline is given in
Listing 4.14 and the source code in Listing 4.15.
The ADC is set to 10-bit conversion, giving an output of 1024 steps:
Internal ADC reference voltage 5.00 V.
Bit resolution 5.00/1024 4.88 mV per bit.
Temperature measurement 10 mV per ºC.
Temperature resolution 4.88/10 0.488ºC per bit.
The temperature is therefore measured to about 0.5ºC. This is quite acceptable, as the
display is precise to only 1ºC. By contrast, if 8-bit conversion were used, the precision
would be only about 2ºC per bit and the display would be misleading.
Figure 4.12: Temperature Sensor System
PIC
16F917
LCD
2 digits
Temp
Sensor
10mV/°C
0°C 500 mV
[TEMP1.C]
AN0
Listing 4.13 Light Switch
///////////////////////////////////////////////////////////////////////
// LIGHTCON.C
// Auto light switch uses comparator inputs on mechatronics board
// Pot 1 adjusted for light switching level.
// Connect: LIGHT to C1–, POT1 to C1+
///////////////////////////////////////////////////////////////////////
#include 16F917.h
void main()
{
setup_comparator(A0_A3_A1_A2); //Setup for PICDEM board
while(1)
{ if(!C1OUT) output_low(PIN_D7); //Switch off LED if lightpot
else output_high(PIN_D7); //Switch on LED if lightpot
}
}
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172. Listing 4.14 Outline of Temperature Sensor Test Program
TEMPDIS
Select MCU 16F917
Include LCD functions
Setup LCD
Setup ADC (10 bits, AN0)
Main loop
Read analogue input (binary 0–1024)
Convert to temperature value (integer)
Convert to BCD digits
Display on LCD (0-99)
Listing 4.15 Temperature Display Source Code
///////////////////////////////////////////////////////////////////////
// TEMP1.C MPB 24-4-07
// Demo program for PICDEM Mechatronics Board
// Displays temperature +1/–0 deg C. Target board link: TEMP-AN0
///////////////////////////////////////////////////////////////////////
#include 16F917.h //MCU header file
#device ADC=10 //Select 10-bit ADC
#include lcd.inc //LCD segment map file
void main() //Start main block
{
int16 intemp; //Input temp from ADC result
float temp; //Decimal result of scaling
int8 distemp, tens, ones; //Display temp and BCD digits
setup_lcd(LCD_MUX14,0); //Initialize 14-pin LCD
setup_adc(ADC_CLOCK_INTERNAL); //Select internal ADC clock
setup_adc_ports(sAN0); //Configure for AN0 input
set_adc_channel(0); //Select AN0
while(1) //Main loop always
{
intemp=read_adc(); //Read analog input
temp=(intemp*0.488)–50; //Convert to degC
distemp=temp; //Truncate to integer
tens=temp/10;
ones=distemp–(10*tens); //Calculate BCD ones digit
lcd_symbol(DigMap[ones],DIG1); //Display low digit
lcd_symbol(DigMap[tens],DIG2); //Display high digit
}
}
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The program needs to convert the input to degrees C by multiplying the input bit count
by the temperature resolution, 0.488ºC per bit. Since the temperature range effectively
starts at 0ºC 500 mV, we must subtract this offset from the calculated temperature. For
example, at room temperature of 20ºC, the sensor output is 500 (20 10) 700 mV.
This converts to a value of 700/4.88 143 (nearest integer).
We check that we see the correct display:
(143 0.488) 50 19.8°C.
Due to rounding down in the program, this displays as 19ºC and changes to 20ºC
only when this input has been exceeded, so the display shows the correct temperature
accurate to 1ºC and 0ºC. A correcting factor of approximately 1/2°C could be
implemented by simply adding 1 to the ADC result to give a display to the nearest
whole degree.
Note that the automatic type conversion incorporated into the complier simplifies the
arithmetic significantly. The type is changed automatically while preserving the value
as far as is possible in the new format. Therefore, a decimal is truncated to an integer by
simple assignment of the value from a float to integer variable.
4.6 PICDEM Temperature Controller
● Specification of temperature controller
● Input and output allocation
● Program outline
The PICDEM mechatronics board will now be used as the hardware platform for a
temperature controller. Using a ready-made board eliminates the need for detailed
hardware design and should be considered if a suitable product is available at a
reasonable cost.
Specification
A temperature controller is required to control a greenhouse or similar outdoor enclosure
at a temperature of 25–30°C using electric heaters and a cooling fan.
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1. Overall function
Maintain target temperature within /2°C, displaying it on the LCD. If the
temperature is within specifications, switch on RunOK indicator; if temperature
difference exceeds 5°C, switch on flash fault indicator.
2. Startup procedure
● Power up the system, reset the fault indicator.
● Display the set temperature on the LCD for operator adjustment.
● Wait for the start input push button.
3. Overall operation
● Switch on the first heater if the temperature is more than 2°C below the target.
● Switch on the other heater if the temperature is more than 5°C below the target.
● Run fans at a speed proportional to the positive temperature difference: If the
fan speed is zero, switch on the fault indicator; if the temperature sensor is out
of range, enable the fault mode.
● If the light level indicates direct sunlight, add a positive offset to the fan speed
in anticipation of an additional temperature rise. If the light sensor is out of
range, enable the fault mode.
The block diagram, Figure 4.13, shows the system I/O requirements.
Figure 4.13: Block Diagram of the Temperature Controller
PICDEM
Mechatronics
Board
Set Temp
Fan
Pulse
Sensor
PIC 16F917
3.5 LCD
ICD
Start
Stop
Reset
OK
Fault
Heater 1
Heater 2
ADC Vref
Temp Sensor
Light Sensor
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Table 4.7: PICDEM Board I/O Allocation for
Temperature Controller (Excluding LCD)
Pin Label Type Board Description
RA0 Tempin Analog in TEMP Range 0–50°C 500–1000 mV
RA1 Lightin Analog in LIGHT Range 0–5 V, needs calibration
RA2 SetTemp Analog in POT1 Range 0.5–1.00V, set target temp
RA3 Vref+ Analog in POT2 Adjusted to 1.024V
RA5 Startin Digital in SW2 Active low, push button, start system
RA6 Stopin Digital in SW3 Active low, push button, shut down
RE3 Reset Digital in SW1 Active low (hard wired) !MCLR
RD4 RunOK Digital out D0 Active high, status indicator LED
RD5 Fault Digital out D1 Active high, status indicator LED
RD6 FanPWM Digital out PWM4 Active high, DC motor, DRIVE 4
RD7 FanEn Digital out N4 Active high, DC motor drive enable
RC5 FanInt Digital in CCP1 DC motor pulse feedback OPT. INT
RD1 Heat1 Digital out N1 Active high, heater 1 on, DRIVE 1
RD2 Heat2 Digital out N2 Active high, heater 2 on, DRIVE 2
I/O Allocation
Once the inputs and outputs required have been established, we can provisionally assign
them to particular pins (Table 4.7), as available in the PICDEM board. The appropriate
links can later be made for testing the application.
Implementation
Output half-bridge drivers 1 and 2 control the heaters. In the final system, these are
interfaced via contactors if the load operates at high voltage. For test purposes, a 6-V
filament lamp is connected to the drive output to represent the heater load. The motor is
operated by drive 4, with the PWM input to the bridge providing speed control. All these
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loads are controlled at the N drive inputs, which operate single-ended in sink mode, since
the current drive is needed in only one direction. The P gates can remain disabled. The
fan speed is controlled using a CCP module in capture mode. This allows low speeds to
be measured accurately.
The temperature sensor is calibrated at 10 mV/°C, with an operating accuracy of 2°C
and offset of 500 mV at 0°C. The temperature range is 0–50°C, so the sensing range is
500–1000 mV. If the second pot is used to provide a reference voltage of 1.024 V, the
10-bit conversion factor is 1 mV per bit, and the temperature is easily calculated in the
program by subtracting 500 from the input.
The light sensor needs to be tested to establish the output level when exposed to sunlight
and a threshold value incorporated into the program, so that the cooling boost cuts
in at an appropriate level. When testing the system, hot and cold air could be applied
to the temperature sensor to check basic functionality, but the set temperature input
provides a more convenient test input. If the temperature at the sensor is constant (room
temperature), adjusting the set input above and below this value has the same effect as the
temperature falling and rising.
If the application functions correctly, when the set temperature is adjusted to the actual
room temperature, neither the heater nor motor output is on. If the set value is increased,
meaning the input temperature is too low, one heater comes on. If increased further,
the other heater comes on. If the set value is decreased, the input appears too high and
the fan comes on. As the set value is further decreased, the fan speeds up. When the set
value is returned to room temperature, all outputs are disabled. If either sensor input is
disconnected (the most likely fault mode), the fault output comes on and all other
outputs are disabled. The same effect is observed if the motor is stalled, simulating a
fan fault.
When the real system is commissioned, the program values may need to be adjusted to
optimize the system response. In this kind of feedback system, the system generally needs
to respond as quickly as possible without showing instability. The loop delay time (wait
for fan) and the PWM calculation might need to be modified accordingly. In commercial
temperature controllers, time constant and gain values are adjustable, so that the system
response can be optimized in situ.
Listing 4.16 outlines the temperature controller program.
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Listing 4.16 Temperature Controller Program Outline
TEMCON temperature control system
Define Initialize
StartIn = RA5 (0/1) Heat1 = RD1 (on/off)
StopIn = RA6 (0/1) Heat2 = RD2 (on/off)
LightIn = RA1 (0–255) FanPWM = RD6 (0-255)
TempIn = RA0 (0-255) FanInt = RC5 (0-255)
SetTemp = RA3 (0-255) Fault = RD5 (0/1)
RunOK = RD4 (0/1) Reset = RE3 (0/1)
Sunlit = 0-255 (calibrate) FanEn = RD7 (0/1)
Startup
All outputs disabled
Loop
Read, store, display SetTemp
While Start button not pressed
Main Loop
Read InputTemp
If InputTemp out of range
Disable outputs
Wait for reset
Flash fault indicator
If (TempIn-SetTemp(–2))
Switch on Heat1
Disable Fan
If (TempIn-SetTemp(–5))
Switch on Heat2
Flash fault indicator
If (TempIn-SetTemp1))
Read FanInt
Calculate fan speed
Calculate PWM duty cycle
Read LightIn
If LightIn out of range
Indicate fault
Disable outputs
Wait for reset
If (LightInSunlit)
Add offset to PWM duty cycle
Modify FanPWM duty cycle
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4.7 PICDEM Board Simulation
● Mechatronics board simulation schematic
● Mechatronics board circuit operation
● Mechatronics board applications
A simulation version of the PICDEM mechatronics board created in Proteus VSM is
provided on the support Web site www.picmicros.org.uk. The ISIS schematic is shown
in Figure 4.14. The circuit has been organized into functional blocks, and some hardware
features are not included to simplify the schematic.
For example, generic drive FETs were used for compactness on the schematic, rather than
the specific devices. It was not necessary to include the circuit of the optical interrupter
interface, since the DC motor and pulse encoder are modeled in VSM as one component.
The RS232 interface is designed to work primarily with a terminal software module
provided with the PICDEM kit and therefore also was not included. Components such as
decoupling and filtering are used only where essential for accurate circuit modeling. The
overcurrent sensing circuit has a simulated input added because variations in the motor
loading cannot be represented; this also allows the operation of this part of the circuit to
be tested independently. The back emf from the DC motor can be modeled by a voltage
source or simple pot if required.
The component numbering is the same as the hardware wherever possible. The circuit
connections between the main blocks are made via terminal labeling in the schematic.
User connections for particular applications can be added as required.
Enable fan
Disable Heaters
Wait 5s for fan to start
If (speed=0)
Indicate fault
Disable outputs
Wait for reset
Else enable RunOK
Always
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179. Figure 4.14: PICDEM Mechatronics Board Simulation Schematic
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Circuit Description
The central component of the PICDEM mechatronics board is the PIC 16F917, whose
main distinguishing feature is the integral LCD drive facility. The 3.5-digit LCD outputs
occupy a large proportion of the available I/O pins, leaving a limited number for the other
peripherals. The digit segments are enabled by appropriate combinations of the segment
and common inputs (see Section 4.2 for details). These are defined in an include file,
which must be added to the application project. Three bias voltages are also required by
the LCD at Vcc, 2Vcc/3, and Vcc/3; these are generated by a simple resistive divider.
The push-button (tactile switch) inputs on the hardware are represented by toggle
switches, so that they can be left in the closed position if necessary when running the
simulation. They can be replaced with buttons if preferred. A bank of active high LEDs
are provided for output monitoring. The temperature and light sensors are modeled as
generic devices, with user control of the set variable. They normally are connected to an
analog input on the MCU, either a comparator or an ADC input.
The drive control logic is also modeled using generic devices for the discrete CMOS
gates but with specific devices for the enable logic. The driver MOSFETs themselves
are generic, so actual device characteristics may not be represented exactly. This is not a
significant issue, since the motor models are also generic.
The PMOSFET is switched on when its gate is taken low, and the NMOSFET is switched
on when its gate is logic high. No additional interfacing is necessary, which is a great
advantage of the FET over other types of current driver, such as bipolar power transistors.
In addition, the FET is voltage operated and input resistance at the gate is very high,
giving negligible loading on the control logic outputs.
The flywheel diodes in the output are added to cut off the back emf from the inductive
motor load when switching off the windings, a standard arrangement with inductive
loads. This high-voltage pulse could otherwise damage the FETs. The specific FETs used
in the actual hardware have Schottky diodes across the outputs, which perform a similar
protection function.
A motor overcurrent is detected by a 0.1- resistor, through which all driver currents
flow to the ground. This generates a voltage of 100 mV at 1 A, and a noninverting
amplifier with a gain of 10 increases this to 1.0 V. This voltage is monitored by a
comparator stage, which has a reference voltage generated by a pair of diodes in series
giving just over 1 V. When this voltage is exceeded, the comparator output triggers the
overcurrent latch, which disables the bridge drivers via their control logic. This latch
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Table 4.8: Bridge Driver Control Logic States
Inputs Outputs Result Drive State
P M N F Pg Ng
X X X 0 1 0 Bridge disabled, both off OFF 1
0 X 0 1 1 0 Bridge disabled (default input) OFF 2
1 1 0 1 0 0 Source on, Sink off SOURCE 3
X 1 1 1 1 1 Source off, Sink on SINK 4
1 0 X 1 0 0 Source on, Sink off SOURCE 5
0 0 X 1 1 0 Bridge disabled, both off OFF 6
Note: Default input (open circuit links) is shown in bold.
needs to be reset via the CLR FAULT push button on power-up or when an overcurrent
condition has been cleared.
Logic functions controlling each half bridge driver have been derived from inspection of the
control logic in the schematics of the mechatronics board in the PICDEM User Manual.
Source FET on: !Pg = P.F.(!(M.N))
Sink FET on: Ng = M.N.F
where
Pg PMOSFET gate (active low),
Ng NMOSFET gate (active high),
N N input from MCU,
P P input from MCU,
M PWM input from MCU,
F FAULT input (disable all outputs).
The operation of each bridge driver deduced from these functions is represented in
Table 4.8, which shows only the significant logic conditions. The full logic table confirms
that the important fact that the FETs are never on at the same time, which would effectively
short out the drive supply. F always disables the output when low (power-up condition from
the overcurrent circuit). For most input combinations, the half bridge is disabled (safe).
When the bridge control inputs are not connected, the P and N inputs are pulled low (0),
the M input pulled high (1) (logic states shown in bold), and the outputs are disabled
(Pg 1, Ng 0, State 2). They are also unconditionally disabled when F is low (Fault
mode, State 1).
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Assuming we start with all inputs open circuit and both FETs off, the bridge is switched
to the Source mode when the P input is taken high (State 3) and to the Sink mode when
N is taken high (State 4). The Sink mode can be used to switch a load connected to the
positive supply on and off or to provide single-ended PWM drive.
For full-bridge operation, P1 and N2, P2 and N1, and M1 and M2 are linked via the six
input links. Drive 3 and 4 inputs are linked in the same way. In this mode, load current
is bidirectional and can be reversed by toggling M with P and N high (States 4 and 5).
States 4, 5, and 6 allow the bridge to be switched between Sink, Source, and Off.
Demo Applications
The mechatronics board simulation represents fixed connections around the MCU by
labeled terminals. Additional connections can be made to uncommitted pins using
the normal wiring tools in ISIS, allowing the demo applications to be tested. Note,
however, that only the full version of ISIS is guaranteed to allow complete control
of the simulation. Therefore, different versions of the mechatronics board schematic
configured for testing particular applications are provided on the support Web site.
Assessment 4
5 points each, total 100
1. Sketch a full bridge driver circuit using PFETs and NFETs connected to a motor,
indicating the current flow for forward motion and the logic state of the FET inputs.
2. Calculate the speed of the stepper motor on the mechatronics board in rev/min if
it is driven at a rate of six steps per second.
3. Derive a formula for the output of the temperature sensor on the mechatronics
board, in the form V f(t).
4. Suggest three disadvantages of using the 3.5-digit parallel LCD compared with
the serial alphanumeric display described in Part 2.
5. Write a statement to display the number ‘8’ on digit 1 on the mechatronics board
LCD, and explain the meaning of each element of the statement.
6. Describe briefly the hardware used to control the speed of a DC motor connected
to a microcontroller.
7. Outline how the position of the stepper motor on the mechatronics board is
controlled and the connections required.
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8. Outline a method for controlling the speed of the DC motor in the mechatronics
board, using Timer1 in the MCU to measure the sensor pulse period.
9. Calculate the delay required in the STEPTEST Program to run the stepper motor
at about 1 rev/sec (full step mode).
10. The temperature sensor on the mechatronics board has a calibrated output, while the
light sensor does not. Explain why the comparator interface is therefore appropriate
for light sensing but the ADC would be preferred for temperature measurement.
11. The temperature at the mechatronics board sensor is 25°C and is converted by the
10-bit ADC with a reference voltage of 2.048 V. Calculate the ADC output value.
12. Write down logic functions for the Source (Pg.Ng) and Sink (!Pg.!Ng)
conditions of the board driver logic in terms of the input variables P, M, N, and F
from the logic states shown in Table 4.8.
13. List the hardware links required for the bidirectional DC motor drive in the
mechatronics board, and explain their significance in terms of switching the
current in the bridge forward, reverse, and off.
14. State the connections required for the stepper motor drive in the mechatronics
board, and list the activation sequence required at the drive logic inputs.
15. State the features of the power MOSFET that make it suitable for use as a current
driver device.
16. Refer to the simulation schematic Figure 4.14 and calculate the output voltage of
the overcurrent amplifier in the mechatronics board simulation circuit when the
test pot is set to its mid-position.
17. Refer to the simulation schematic Figure 4.14 and explain briefly how the
overcurrent latch functions.
18. Explain briefly why a PMOSFET and an NMOSFET are needed in each half-
bridge driver stage.
19. Outline how to set up the mechatronics board to control the speed of the DC
motor in one direction only, and state the required output from the MCU.
20. Study the setup for stepper motor driving in full-bridge mode; and by using the
drive logic functions, determine the winding activation sequence, in terms of the
current flow between drive terminals 1, 2, 3, and 4.
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Assignments 4
To undertake these assignments, install Microchip MPLAB (www.microchip.com), Labcenter
ISIS Lite (www.proteuslite.com), and CCS C Lite (www.ccsinfo.com). Application files may
be downloaded from www.picmicros.org.uk. Run the applications in MPLAB with Proteus
VSM selected as the debug tool. Display the animated schematic in VSM viewer, with the
application COF file attached to the MCU (see the appendices for details).
Assignment 4.1
Download the mechatronics board simulation file PICDEMboard.DSN and attach the
program test.cof. Check that the simulation runs correctly, causing the outputs at Port
D to display a binary count. Modify the delay count and confirm that the output timing
changes accordingly.
Assignment 4.2
Download the PICDEM mechatronics board simulation file PICDEMdcmotor.DSN and
attach the program motorsim.cof. Check that it runs correctly, displaying the motor
revs completed on the display after the input switch has been operated. Modify the
program to measure the time interval between pulses from the motor sensor and display
the speed in rev/sec. To implement this, measure the pulse interval in microseconds using
Timer1 (maximum count 65 ms) in Capture mode, MCU clock 4 MHz. This gives
the time taken for half a rev in microseconds, th, and the speed can then be calculated,
in rev/sec 106/2th. For example, if the speed is 3000 rpm (probably exceeding the
maximum achievable by the motor), we should see 50 rev/sec on the display. The value
of th will then be 10 ms, a count of 10,000 in Timer1. We can see from this that the
minimum speed measurable is about 10 rev/sec. Use a suitable prescale value to extend
this value to less than 1 rev/sec, and modify the program to improve the precision of the
speed measurement to 0.1 rev/sec.
Assignment 4.3
A temperature controller program is required for the mechatronics board that implements
a cooling system. The DC motor has a fan attached, and the controller increases the fan
speed when the temperature increases. Connect up the mechatronics board for PWM
control of the DC motor. Write a cooling program that reads the temperature sensor and
modifies the motor speed accordingly. Demonstrate the application in simulation or
hardware as facilities allow.
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PIC16 C Applications and Systems
5.1 PIC16 C Application Design
● Block diagram
● Program outline
● Debugging and testing
Formal design methods recommended for engineering projects may need to be applied in
the professional design environment. Here, some basic methods are outlined as a starting
point; these allow new applications to be developed with some degree of consistency and
help communicate project concepts and design details clearly in reports and presentations.
Hardware Design
The block diagram is an effective way to show the general form of a microcontroller
application design, and examples are seen throughout this book. Some simple rules are
used to represent system blocks and their input and output signals:
● The direction of signal flow is represented by an arrowhead.
● The TTL level digital signal is the default (default arrow) style.
● Other switching levels (e.g., RS232 line) are indicated by labels.
● The analog voltage range is indicated by a label and arrow style.
● Parallel data are represented by a block arrow.
● Analog signals are represented as a simple waveform.
P A R T 5
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The block diagram (Figure 5.1) is easily constructed using only the drawing tools in a
standard word processor. The example in Figure 5.1 might represent an analog-to-digital
converter chip, with an “end of conversion” output.
Once a block diagram has been created, defining the inputs and outputs of each block, a
circuit schematic can be derived from it.
Software Design
The application program can be designed using various methods. A flowchart shows
the overall program sequence in a visual manner and is good for illustrating simple
program sequences. However, for C programs, some form of structured pseudocode
is recommended, where the main program is outlined as a text file, which can then be
converted directly to source code. Examples are again found throughout this book; the
general content is described in Listing 5.1.
After the application program source code has been created in the MPLAB text editor,
it can be compiled to generate the project file set. This includes the MCU machine code
HEX file and the COF file, which incorporates the hex file with additional debugging
information. It is necessary to have all the project files in the same folder, making copies
of the resource files as necessary. All applications need an MCU header file, such as
16F877A.H.
The application source code, MCU header file and any other files to be included or used
must be attached to the project in the project file window. The application can then be
built and the HEX machine code file produced. This is downloaded to the target system to
operate the application in hardware.
Application Debugging and Testing
The application program is tested and debugged in several stages. The main types of
errors and the tools for detecting them are outlined next.
Text Box with
Functional
Description
Analog
Voltage Parallel Data Output
Single TTL Output
0 – Vm
Figure 5.1: Block Diagram Conventions
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Syntax errors are mistakes in the source code, such as spelling and punctuation errors,
incorrect labels, and so on, which cause an error message to be generated by the compiler.
These appear in a separate error window, with the error type and line number indicated so
that it can be corrected in the edit window.
When the program is successfully compiled, it can be tested for correct function in the
target hardware so that any logical errors can be identified. However, it is preferable to
test it in software simulation mode first, as it is quicker and easier to identify errors in the
program sequence. Two simulation methods are available here, MPSIM and Proteus VSM.
MPSIM is the simulator provided with MPLAB. It allows the program source code to
be run, stopped and stepped, and breakpoints set. The registers and source variables may
be inspected at each step. When debugging C programs, breakpoints are the most useful,
Listing 5.1 General Control Program Outline
PROGNAME.C ///////////////////////////////////
Program header information
Author, date, version etc
Include MCU header file
Include function library files
Include user source files
Use function library files
Define constants
Declare global variables
Declare function prototypes
Main block //////////////////////////////////
Initialization sequence
Initialization function calls
etc
Main loop
Sequences
Function calls (level 1)
etc and repeat
Function block //////////////////////////////
Initialization sequence
Process sequences
Function calls (level 2)
etc and return
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while stepping is more useful in assembly language. The program sequence and variable
values are monitored and errors identified when the results obtained do not agree with
those expected. Error information is provided principally in tabular form.
By comparison, the Proteus VSM debugging environment has significant advantages.
The animated schematic gives a much more immediate indication of the overall program
function. Interactive input and output devices operate in real or simulated time. The
source code and breakpoints can be displayed.
In addition, if the VSM viewer is run from within MPLAB, the progress of the program
can also be monitored simultaneously in MPSIM. Therefore, the more detailed debugging
tools in MPSIM can be run alongside VSM and the most appropriate selected for any
debugging task. The simulated hardware design is thus tested in conjunction with the
MCU firmware (cosimulation), allowing circuit modifications at an early stage and
hardware-software interaction to be studied on screen. When the program is eventually
downloaded to the real hardware, it is now far more likely that it will work the first time.
The VSM Viewer is invoked from the debug tools menu in MPLAB, and the program is
attached and tested. However, if circuit modifications are needed, VSM must be opened
separately to run alongside MPLAB, so that the full set of ISIS schematic edit tools and
component models are available. VSM still accesses the same COF file, so both software
and hardware changes can be tested. More details on interactive debugging are given in
Appendices A, B, and C.
5.2 PIC16 C Temperature Controller
● Basic system
● Software design
● Implementation
In this section, the software design principles just outlined are applied to a typical
application, a temperature control system. The schematic of the demo hardware is shown in
Figure 5.2. The TEMP pot represents a temperature sensor that outputs a voltage of 0–5V.
If a scaling of 100mV/°C is assumed, the range is 0–50°C, with 2.5V representing 25°C.
System Operation
The sensor is connected to AN1, the ADC channel 1. A SET pot provides the reference
temperature for the system. If the measured temperature is below the set level, a heater,
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represented by the filament lamp output, is switched on. If it is above the set value, a
cooling fan switches on instead, represented by the DC motor.
To avoid the outputs “chattering” at the switching point, due to input noise, switching
hysteresis should be incorporated into the control sequence, meaning that the switching
level when the temperature is rising is higher than when the temperature is falling.
The temperature is displayed on the serial LCD as well as the status messages Heater ON
or Fan ON. The program structure ensures that the correct message is displayed during
the changeover phase.
Software Design and Implementation
The process of designing the software can be aided by writing a program outline. The
main structures and sequences are summarized using suitable layout and operational
descriptions.
A typical problem to be overcome is that the displayed message must agree with the output
status in the presence of hysteresis. Therefore, an output status flag (variable type int1) is
used to record the current output status. This flag is then tested by the conditional output
statement. Note that the switching levels can be modified to suit the application. In the
code shown (Listings 5.2 and 5.3), the upper switching level is 20 steps above the lower.
RB0/INT
13
SET TEMP
TEMPERATURE CONTROLLER
temcon.dsn
TEMP
100 mV/deg C
33
34
35
36
37
38
39
40
15
16
17
18
23
24
25
26
VDD
LCD1
L1
6V
RXD
VSS
MILFORD-2X16-BKP
19
20
21
22
27
28
29
30
RA0/AN0
RB1
RB2
RB3/PGM
RB4
RB5
RB6/PGC
RB7/PGD
RC0/T1OSO/T1CKl
RC1/T1OSI/CCP2
RC2/CCP1
RC3/SCK/SCL
RC4/SDI/SDA
RC5/SDO
RC6/TX/CK
RC7/RX/DT
RD0/PSP0
RD1/PSP1
RD2/PSP2
RD3/PSP3
RD4/PSP4
RD5/PSP5
RD6/PSP6
RD7/PSP7
RA1/AN1
RA2/AN2/VREF⫺
RA3/AN3/VREF⫹
RA4/T0CKI
RA5/AN4/SS
RE0/AN5/RD
RE1/AN6/WR
RE2/AN7/CS
PlC16F877 U1
OSC1/CLKIN
OSC2/CLKOUT
MCLR/Vpp/THV
14
1
2
3
4
5
6
7
10 k
8
9
10
10 k
Figure 5.2: Temperature Control System
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In a real system, the interfacing needs to be further developed. The temperature sensor
is likely to need an amplifier, perhaps with voltage-level shifting. The heater and fan
need a relay or contactor to operate the final load, with the relay requiring a transistor
interface or current driver. Details of interface design can be found in Interfacing PIC
Microcontrollers by the author.
5.3 PIC16 C Data Logger System
● BASE board hardware
● Application design
● Program outline
Since this book is concerned mainly with software development, off-the-shelf hardware,
such as the PICDEM mechatronics board featured in Part 4, is very useful. This is
Listing 5.2 Temperature Control Program Outline
TEMCON
Initialize
MCU 16F877A
ADC 8 bits, Inputs RA0, RA1
RS232, Output RD0
Loop
Delay 500ms for display
Read Set Pot 0-255
Read Temp 0-255
Scale Temp for display
Display Temp on LCD line 1
If Temp below lower limit
Switch ON Heater
Switch OFF Fan
If Heater is ON
Display on LCD line 2
If Temp above upper limit
Switch OFF Heater
Switch ON Fan
If Fan is ON
Display on LCD line 2
Always
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Listing 5.3 Temperature Controller Source Code
/*
TEMCON.C MPB 27-3-07
Temperature controller demo. Target simulation system: TEMCON.DSN
***********************************************************************/
#include 16F877A.h
#device ADC=8 // 8-bit conversion
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_D0, rcv=PIN_D1) // Display output
void main() //*****************************************************
{
float refin, numin, temp;
int1 flag;
setup_adc(ADC_CLOCK_INTERNAL); // Setup ADC
setup_adc_ports(ALL_ANALOG);
for(;;) // Repeat always
{
delay_ms(500);
set_adc_channel(0); // Read ref. volts
refin = read_adc();
set_adc_channel(1); // Read temp. volts
numin = read_adc();
temp = (numin*50)/256; // Calc. temperature
putc(254); putc(1); delay_ms(10);
printf( Temp = %3.0g ,temp); // Display temp.
putc(254); putc(192); delay_ms(10);
if (numin(refin-10)) // Temp. too low
{ output_high(PIN_B1); // Heater on
output_low(PIN_B2); // Fan off
flag = 1;
}
if (flag==1) printf( Heater ON ); // Status message
if (numin(refin+10)) // Temp. too high
{ output_low(PIN_B1); // Heater off
output_high(PIN_B2); // Fan on
flag = 0;
}
if (flag==0) printf( Fan ON ); // Status message
}
}
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reflected in real applications by the use of standard hardware such as PC-compatible
boards as the platform for a wide range of applications.
BASE Board
A general purpose board with a typical selection of peripherals attached to a PIC
16F877A is described here. This design was originally developed to demonstrate
hardware interfacing techniques. The PIC 16F877 BASE (basic application and system
evaluation) board incorporates six analog inputs, a 12-button keypad, a parallel 16 ⫻ 2
character LCD, 16k serial memory, an RS232 port, and ICD programming connections.
The block diagram is shown in Figure 5.3, the schematic in Figure 5.4.
Here, the board is used as a data logger. It records input analog voltage levels at timed
intervals and stores this data for later uploading to a host PC. The PIC 16F877 has
eight 10-bit analog inputs, but to keep the demo system simple, 8-bit conversion is
used. The reference voltage applied to RA3 is 2.56 V, which gives a resolution of
2.56/256 ⫽10mV per bit and a precision of 100/256 0.4%.
The reference voltage and a test input occupy two of the analog inputs, so six are available
for connecting to an external target system. Typically, the inputs are connected to analog
sensor inputs, measuring temperature, position, strain, and other physical variables from
suitable sensors. Another possibility is that the target system is an analog board whose
performance is being evaluated by measuring the circuit voltages under test conditions.
PIC
16F877A
MCU
X7
X7
X7
User I/O
(Digital or
Analog)
Serial
Memory
X12
Keypad
LC Display
X3
Reset
Clock
4 MHz
Test Input
0–2.5 V
LED
Buzzer
RS232
ICD
Vref ⫽ 2.56 V
Figure 5.3: BASE Board Block Diagram
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Figure 5.4: BASE Board Circuit Diagram
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The measured values are stored in an I2
C serial flash memory chip, which retains the
data when powered down. The driver routines for this device are demonstrated in section
3.6. The data can be transferred later to a host PC or other data terminal via the RS232
interface. A driver chip is fitted to convert the data to line voltages.
The board has a simple keypad, where operational parameters, such as the sampling
interval, can be input during initialization or the mode of operation toggled between
“logging” and “uploading.” Scanning a keypad is described in section 2.6 in connection
with the calculator demo application.
Listing 5.4 Program Outline for Data Logger
LOGGER
Initialize
Delays
Analogue inputs
UART port
I2C port
Interrupts
Main
Set logging interval
Select active analogue inputs
Enable interrupts
Wait
Interrupt Routines
Timeout
Restart timer
Read selected analogue inputs
Store in external EEPROM
Display channels and input voltages
Return from interrupt
Zerokey
Disable timer
Display 'Logging Stopped'
If Starkey
Restart logging
If Hashkey
Send data via RS232
Display 'Sending data'
Return from interrupt
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The parallel LCD is used to display status messages and data as they are sampled. It
is useful to compare it with the serial LCD described previously, as parallel access is
generally faster, particularly when bit maps are used for graphics in more sophisticated
applications. The 8-bit ASCII and control codes must be sent as 4-bit nibbles from
RD4-7, with RD1 acting as the register select (RS) input and RD2 generating the data
strobe (E). More details are provided on driving the parallel LCD in Interfacing PIC
Microcontrollers, by the author. Alternatively, the manufacturer’s data sheet can be
consulted for the necessary control codes and timing information.
Program Outline
As can be seen in the program outline (Listing 5.4), the application is largely interrupt driven.
The timer interrupt is the simplest way to generate a regular event, in this case, sampling at
fixed intervals. The 0 key is used to interrupt the logging process, so it might be desirable to
reassign the input from column 2 of the keypad to RB0, the primary interrupt input. Logging
is restarted using the star (*) key and data upload initiated using the hash (#) key.
5.4 PIC16 C Operating Systems
● Polling
● Interrupts
● RTOS
As microcontroller operating programs become more complex, consideration must
be given to the best method of organizing the program response to input, memory
management, and output timing. Three main methods are used to handle input and output
events, which after all, is the primary requirement of a real-time system. In order of
complexity, they are I/O polling, interrupts, and the real-time operating system (RTOS).
Polled I/O
This is the easiest, and may be considered the default, method of input and output,
where operations are simply scheduled as part of the main loop. It is seen in most of
the examples in this book, because they have been deliberately kept simple. The basic
principle is illustrated in Figure 5.5.
This option is fine if the delay that occurs between input signal and output response is
not critical to the correct overall operation of the system. The time taken to complete
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the input processing may vary significantly, depending on the input data or programmed
options within the loop. For example, a test on the data may result in an optional
sequence being executed, or not, depending on the value. In fact, this is pretty much
inevitable in most real programs.
However, it is often important for the input and output timing to be more predictable. Take
the example of motor speed control. In small DC motors, this is usually implemented by
pulse width modulation, as discussed in section 4.3. The output is switched on and off over
a regular cycle, the proportion of “on” to “off” time determining the average motor current
and hence the speed. To achieve accurate control, the shaft speed must be measured,
usually by a pulse encoder. The input pulse interval must be measured and the PWM duty
cycle adjusted accordingly. It is just about possible to do this using a polling process
(see PIC Microcontrollers, An Introduction to Microelectronics by the author, 2004), but a
more elegant solution can be implemented using interrupts.
Interrupts
As we have seen in Section 2.9, interrupts are internally or externally generated
asynchronous hardware signals that force the processor to stop its current (background)
task and carry out the interrupt service routine (ISR), a higher-priority (foreground) task.
The processor “context” (current register contents and status) must be saved and the
current program address stored on the stack so that the background task can be resumed
when the ISR has finished.
Let us see how this can be applied to the motor controller, assuming we are using a 16F877
MCU (Figure 5.6). The input pulse period can be measured using one of the hardware
timers. Since Timer2 is designed to provide PWM mode, Timer1 can be used to monitor
Process Input
Write Ouput
Read Input
Initialize
START
Figure 5.5: Polled I/O Process
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the input, working in Capture mode. The counter/timer register is fed from the system
clock to measure absolute time intervals, and the count is stored when the input changes.
The pulse period can then be worked out and this result compared with a target value,
which represents the required period (hence speed). If it is too long (speed too low),
the motor speed is increased by increasing the PWM duty cycle in Timer2. If too short
(speed too high), the duty cycle is reduced. An interrupt is generated by Timer1 when the
count is captured; the ISR modifies the output duty cycle as required, and the controller
then waits for the next interrupt to occur.
If the program uses multiple interrupts, one ISR may be interrupted by another. The
interrupts may need to be assigned an order of priority, so that a less important task does
not interrupt a more important one. When the higher-priority ISR is being executed, the
lower-priority interrupt can be disabled, or masked, until it is finished. In more complex
programs, numerical levels of priority can be assigned, with higher priorities taking
precedence. Unfortunately, the 16 series PIC is not well suited to this, as it does not have
a built-in priority system, unlike more powerful processors. Further, the different interrupt
sources have to be identified explicitly by a user routine.
An operating system (OS) provides an alternative to interrupts as a means of providing
a more predictable time response in the microcontroller system but again is typically
implemented in the higher-power MCU type, such as the PIC18 or 24 series.
Nevertheless, to point the way ahead, the principles are outlined here.
PC Operating System
The most well-known example of an operating system is Microsoft Windows®
. Why is
this needed in PC-type computers? The answer is simply the complexity of the software
START
ISR
High-
Priority
Task
Return from
Interrupt
Interrupt
Initialize
Background
Task
Figure 5.6: Basic Interrupt Operation
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compared with a microcontroller. The operating system provides a collection of the
numerous program components required to run the computer. Each peripheral interface
has its own driver (keyboard, screen, disks, mouse, network, etc.) plus modules for
memory management and general system control.
Therefore, the PC needs a more sophisticated task management system. A lengthy
process, such as printing or disk access, cannot be allowed exclusive use of the system
resources. If the processor ignores the keyboard completely while downloading a large
file from the Internet, the user cannot access the system to do something more urgent. In
addition, the OS has to be multitasking; that is, it must allow several operations to appear
to be running simultaneously, such as allowing you to keep writing while printing. We
also want to switch quickly between tasks by keeping more than one window open at a
time, which means keeping multiple tasks loaded in memory. For example, while running
the examples in this book, we need to have MPLAB and Proteus open at the same time,
plus maybe a data PDF and the word processor.
Multitasking is essentially achieved by time slicing. Each apparently concurrent task is
allowed to run for a given time interval, say 100ms, then execution switches to another.
Priority can be assigned, so that, for example, one Internet data packet is picked up and
stored in memory before the next arrives and overwrites it in the network data buffer.
Therefore, the OS is designed so that multiple tasks appear to run smoothly together and
with the right priorities.
The PC is essentially a batch processing system; that is, the timing of the major tasks is
not critical. If a word-processing task is delayed for a few milliseconds, it is not apparent
to the user and not significant in terms of overall system effectiveness. On the other hand,
the timing of events in so-called real-time systems must generally be highly predictable.
When an input is received, it must be processed and the output generated within a known
time frame. The point is obvious if one considers an example such as an aircraft flight
control system or automobile engine controller. To manage complex control system
software, we may need a real-time operating system.
Real-Time Operating System
The principle of operation of a simple RTOS, as implemented by CCS C, is shown in
Figure 5.7. The program is divided into separate tasks, which are executed in turn. A timer
interrupt causes the task switching, but interrupts are otherwise not used. When a task is
suspended, its context (file register state) is saved and restored when it is restarted the next
time around. In this way, multiple tasks are executed in rotation and can appear to execute
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simultaneously, and the I/O timing is more predictable. More sophisticated systems
incorporate task priority and implement more complex task management strategies.
A blank program is shown in Listing 5.5 to illustrate how CCS C implements the RTOS. The
MCU used is an 18F452, which is the 18 series equivalent to the 16F877 (CCS C supports
RTOS for only 18 series PICs and above). The delays in the RTOS are implemented using
the standard function, where the MCU clock rate has to be specified (20MHz).
The directive #use rtos()indicates to the compiler that this program uses the RTOS
structure. It then expects some task definitions to follow and the main block to contain
the statement rtos_run()
. The hardware timer used to produce the timer interrupt that
triggers task switching is specified as an argument of the directive, Timer0 in this case.
The “minor cycle” defines the maximum time for which the task runs. Each individual
task execution rate must be a multiple of this time.
The task definitions follow. Each is preceded by the directive #task, so that the
compiler knows this is an RTOS task and not a standard function definition. The rate
specifies how often the task executes (e.g., once per second for Task 1), and max is the
maximum time allowed for this task. The task block is then defined as a sequence of
statements in the same way as a standard function, but bear in mind that its execution can
be suspended and restarted at intervals defined by the RTOS.
All that remains then is to start up the RTOS in the main block, and the tasks are
executed in turn, with the frequency and duration specified for each. The CCS
implementation is classified as a cooperative, multitasking RTOS. This means that the
tasks return control to the scheduler voluntarily to allow the next to run. A set of functions
START
Initialize OS
Task 1
Task 2
Task 3
Task N
Timer Interrupt
Timer Interrupt
Timer Interrupt
Timer Interrupt
Figure 5.7: Basic RTOS Operation
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are supplied that allow the tasks to work together for optimum effect. For example,
rtos_enable(task1) and rtos_disable(task1) allow tasks to be selectively
enabled and disabled. The function rtos_yield() allows the task to return control to
the scheduler when finished. Some functions allow status information and messages to
passed between tasks and the progress of the tasks to be monitored.
The RTOS is implemented with a total of only 13 functions and directives (see the CCS
C Compiler Reference Manual). A good general explanation of RTOS principles and
types can be found in the Salvo RTOS User Manual, Chapter 2, from Pumpkin Inc.
(www.pumpkininc.com).
Listing 5.5 Blank RTOS Program
// RTOS1.C
// Minimal blank RTOS program
///////////////////////////////////////////////////////////////////////
#include 18F452.h // Define MCU
#use delay(clock=20000000) // Define clock rate
#use rtos(timer=0,minor_cycle=100ms) // Define RTOS timing
// Task functions /////////////////////////////////////////////////////
#task(rate=1000ms,max=100ms) // Define first task
void task1()
{
// Task1 statements...
}
#task(rate=500ms,max=100ms) // Define another task
void task2()
{
// Task2 statements...
}
#task(rate=100ms,max=100ms) // Define last task
void task3()
{
// Task3 statements...
}
// Main function //////////////////////////////////////////////////////
void main()
{
rtos_run(); // Start RTOS scheduler
}
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5.5 PIC16 C System Design
● Hardware selection
● Software design
● System Integration
We have seen how to get started with building PIC microcontroller systems programmed
in C. Simple examples have been used to illustrate the basic principles, so we now need to
look at some issues relating to more complex microcontroller-based systems. Numerous
texts are available, written by experienced and knowledgeable engineers, that discuss
the finer points of real-time system design, so the intention here is to introduce the some
basic concepts to help the reader to move toward a further understanding of real industrial
applications. Another objective of this section is to review some relevant factors in the
selection of the best combination of hardware, programming language, and development
tools for any given microcontroller product design.
Hardware Selection
There is a range of related devices around which embedded systems may be designed,
including a
● Microcontroller (MCU)
● Microprocessor (CPU)
● System on a chip (SoC)
The conventional microprocessor system embodies the traditional approach, where a central
processing unit, memory, and peripherals can be put together to meet the requirements of a
particular application as precisely as possible. Designing a custom-made CPU system is a
relatively expensive option, and such an extensive range of other options are available that
the conventional CPU-based system may be needed for only highly complex, specialist
systems or where a low-cost, standard board such as the PC motherboard can be easily
adapted. The discrete microprocessor does, however, allow multiprocessor systems to be
designed that typically use shared hardware resources, especially memory. Current standard
processors typically incorporate features to support multiprocessor operation, and the dual
core processor is currently becoming standard in PCs.
The SoC takes the concept of the microcontroller to the next level. It is, in effect,
a configurable microcontroller, where the designer has control over the internal
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arrangement of the hardware elements. Using a dedicated design system, the processor
core is selected and the required memory and peripherals added. These hardware
elements are supported by corresponding standard drivers provided as part of the
package. With a complex interface, such as USB, for example, the provision of a standard
protocol stack (software layers, not a hardware stack) is essential. The design can be fully
tested in software, in the same way that a PIC program can be tested in MPLAB. Only
when finally verified is the design fabricated by the hardware supplier.
If a design is to be created from scratch, then the most appropriate type of system may be
selected from the three main options listed previously. However, this choice is unlikely
to occur in isolation; factors such as the previous experience of the design team, existing
company products, and so on are significant. Nevertheless, the designer should keep an
open mind as far as possible and needs to keep up with a rapidly developing technology
in the embedded systems field to make the right choice—not easy.
Microcontrollers
A designer who has a store of expertise using a particular microcontroller type and
development system will need a good reason to look elsewhere for a solution. Gaining
similar expertise in another system takes time and resources, and any change must also
take into account the future strategy of the company or design group.
The PIC family may be our first choice for the following reasons:
● Low cost
● Simplicity
● Good documentation
● An established market
● A development system provided
● Third party support
The PIC is well suited to the learning environment as it was originally pitched at the low-
end (high-volume, low-complexity) market and is well supported by third party products.
Therefore, the assumption implicit in this book is that the PIC is the best starting point,
even if the learner is later to progress to other processor types. At the time of this writing
the main alternatives are Atmel (AVR), Freescale (Motorola), STMicroelectronics,
Hitachi, Philips, and National Semiconductor.
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We can approach hardware selection on the basis of the choice offered within the PIC
range, which was outlined in section 1.1. Some of the main features to consider are
● The number of I/O pins.
● The interface types.
● The program memory capacity.
● The RAM capacity.
● The operating speed.
● The power consumption.
We assume that adequate development system support and driver libraries are available.
A logical approach to design is to select a chip that has spare capacity in relation to the
draft specification. The application can be prototyped in simulation mode without penalty
using an overspecified device. When the I/O, memory, and peripheral requirements
finally are established, a chip can be selected for hardware implementation that meets the
specification at minimum cost.
The anticipated scale of production is also a factor. The cost of each individual unit
produced becomes more critical as the scale of production increases. On the other hand,
the firmware can be reproduced at effectively no cost, unless variants are required. If we
assume a fixed cost, a, for design development (hardware and software) and each board
costs b to produce, the cost per unit is given by
y a x b
⫽ ⫹
/
where x is the number of units produced. The fixed costs are divided by the number of
boards produced. So, if the development costs are, say, 1000 units of currency (a ⫽ 1000)
and the production cost 100 per board (b ⫽ 100), a curve showing the cost per board as
the volume of production is increased is obtained, as seen in Figure 5.8. We can see that
the cost per board is initially high, falling away and leveling off as the production volume
increases.
Hardware Design
Taking the hardware design criteria in turn, we can consider some of the relevant factors
in getting started with a design, assuming an agreed-on initial specification. Having said
this, it is useful to know how much flexibility is allowed in meeting the specification,
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because a disproportionate cost might be involved. It may be acceptable to reduce the
performance to reduce costs, for example, reducing the precision of analog measurements
or the frequency range of a signal output.
The cost of the microcontroller tends to increase with the number of I/O pins, so it is
probably a good idea to look for ways to reduce the pin count. One example we saw in
previous sections is to use a serial LCD instead of a parallel one. The serial type requires
only 1 output, while the parallel LCD seen earlier needs 7, or possibly 11 if 8-bit data
are used. Certainly the serial interface should be considered the default choice, and the
parallel used only if high-speed access to the display is needed. The serial link can also
be physically longer.
Serial access sensors are becoming more common, where the data are sent to the MCU in
serial form, rather than as an analog signal. We saw that any pin can be used as an RS232
port, because CCS C provides a driver that generates the required interface purely in
software. This means dedicated analog ports may not be necessary, giving greater flexibility
in the choice of MCU. On the other hand, the sensor is likely to be more expensive.
Program memory capacity requirements are not easy to anticipate before the software has
been finalized. C programs generally need more memory than assembler, so the choice
of language is important. This factor is considered further later, but for now, suffice it to
say that memory requirements expand rapidly with program complexity. As regards RAM
1200
1000
800
600
400
Cost
per
board
Number of boards
0 5 10 15 20 25 30 35
200
0
Figure 5.8: Production Cost
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requirements, the PIC is strictly limited, as the only onboard RAM consists of spare
file registers. External data memory may well be necessary, as in our data logger. An
alternative type of MCU could even be necessary for data-intensive applications.
The PIC scores well on operating speed, however. The 16 series devices can generally
run at 20MHz, with the 18 and 24 series running at 40MHz. The clock speed does
affect the power consumption, as the current consumed is proportional to the switching
rate in CMOS devices. Low-power MCUs are an important ongoing development in
microprocessor technology. Reduced operating voltages (e.g., 3.3-V supply) are also
increasingly used to reduce power dissipation. Power consumption is not one of the
operating parameters normally predicted by simulation, so a real hardware prototype may
be needed to finally specify the power supply. Obviously, power consumption is even
more critical in battery-powered systems.
Software Design
There are two main options for creating the system firmware for low-complexity
embedded systems: assembly language or C. There are other user-friendly programming
options aimed primarily at learners, such as software that allows C code to be generated
from a flowchart (see Appendix D). A wider range of high-level languages and
proprietary development systems are available to support more advanced processors.
In general, assembly language is used for simple programs and those where direct access
to control registers or speed is critical. Certainly, using assembler requires an intimate
knowledge of the MCU architecture and is an essential tool for the practicing embedded
engineer. If necessary, assembly language blocks of code can be embedded within a C
program.
However, the premise of this book is that there are good arguments for starting with C. Less
detailed hardware knowledge is needed, and programming is simplified. It is also a universal
language, whereas each MCU type has its own assembly language. Used in conjunction
with a user-friendly simulator, such as Proteus, useful applications for any microcontroller
type can be created with a minimum of experience. The availability of a comprehensive
set of peripheral drivers is also very helpful, as provided by CCS C. However, the main
advantage is that C is by far the most widely used high-level language for embedded
systems and can be applied by all embedded engineers, from beginner to expert.
The overall structure of the embedded firmware is determined by the complexity and, to
some extent, the hardware features of the host MCU. A simple program can use polled
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I/O in assembler program. If the chip has an interrupt structure that allows task priority
and timing to be adequately managed, then interrupts can be used in assembler or C. The
RTOS approach may well be the best solution for more advanced applications; this is the
next stage in microcontroller system design, to which I hope the reader will be able to
progress because of the system design concepts outlined in this book.
There is never a perfect solution to the embedded design challenge, but we can try for the
best one that lies within our own limits of experience and enjoy the challenge it presents.
Assessment 5
5 points each unless otherwise stated, total 100
1. Explain why hysteresis is useful in processing switched inputs.
2. Write two C statements that select analog input AN1 and read it, and explain
briefly why the variable comes first in the read statement but is given as the
function argument in the select statement.
3. Draw a block diagram of a simple temperature control system, consisting of a
temperature sensor, heater, fan, start and stop buttons, and status indicators for
“running” and “temperature OK.” (10 points)
4. Write a basic program outline for the system described in Question 3 which has a
single fixed operating temperature and no hysteresis. A polling loop will wait for
the start button to be operated, while the stop button will shut down the system
via the MCU reset input.
5. Explain briefly why analog inputs, serial flash ROM, and a serial data link are
useful features of data logging system hardware.
6. Explain briefly how the use of a timer interrupt allows an accurate data logging
interval to be more easily implemented than simple input polling.
7. Explain briefly the meaning of interrupt priority.
8. Compare briefly the different features of a standard PC operating system and an
RTOS.
9. Explain briefly the significance of each part of the CCS C RTOS task definition
directive #task(rate=500ms,max=100ms).
10. Explain briefly the main difference between a microprocessor and
microcontroller-based hardware system.
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11. Explain briefly the main advantage of a SoC when compared to a conventional
microcontroller.
12. State five criteria for selecting a microcontroller type or family.
13. State five criteria for selecting a microcontroller for a given application.
14. Explain briefly why the cost of a microcontroller application prototype is
relatively high, but the cost per unit reduces as more systems are produced using
that design, and sketch a curve that illustrates this fact.
15. Compare briefly the merits of a serial alphanumeric LCD module and the DMM
display used in the PICDEM mechatronics board.
16. Discuss briefly the factors that affect power consumption in an embedded system
and how to evaluate it.
17. Explain the advantages of using C for embedded applications. (10 points)
Assignments 5
To undertake these assignments, install Microchip MPLAB (www.microchip.com), Labcenter
ISIS Lite (www.proteuslite.com), and CCS C Lite (www.ccsinfo.com). Application files
may be downloaded from www.picmicros.org.uk. Run the applications in MPLAB with
Proteus VSM selected as the debug tool. Display the animated schematic in VSM viewer,
with the application COF file attached to the MCU (see the appendices for details).
Assignment 5.1
Download the project TEMCON and check that it runs correctly in MPLAB with Proteus
VSM viewer. Modify the program to display warning messages when the temperature is
more that 3°C above the upper switching level (TOO HOT) or more that 3°C below the
lower switching level (TOO COLD).
Assignment 5.2
Design a controller for a small hot and cold drinks machine, aimed at the domestic
market. Write a specification based on your own understanding of the typical
requirements of such a machine, draw a block diagram showing the interfacing required,
and outline a control program which can be implemented in C. Predict the input, output,
and memory requirements and select a PIC microcontroller (www.microchip.com) which
provides the features required for this application at minimum cost.
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Hardware Design Using ISIS
Schematic Capture
Proteus VSM is an interactive electronics design package from Labcenter Electronics that
allows analog, digital, and microprocessor circuits to be subjected to virtual testing before
the creation of a PCB layout for the construction of real hardware. ISIS is the schematic
capture package, and ARES is the layout package.
The circuit is entered directly onto the schematic by selecting components from a library
of parts, which have associated mathematical models (e.g., V ⫽ IR for a resistor).
When completed, the wiring schematic is converted to a set of nodes connected by
components, represented by a set of simultaneous equations derived from the model for
each component. The network is solved for any given set of inputs and the outputs are
displayed via active on-screen components, virtual instruments, or charts.
The microcontroller is simulated on the basis of its internal architecture and the specific
program being executed, which must be attached to complete the model. In our case, the
program is written in C and the COF file produced by the compiler attached to the MCU.
This file contains the program machine code and some additional information to help
with debugging the program. ISIS allows the source code and variables to be displayed
so that the program operation can be studied step by step and any functional errors
corrected.
Design Specification
The starting point for an electronics design is a specification, which should state clearly
the system performance requirements. Our example project is called BAR1 (Figure A.1).
A P P E N D I X A
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This is used as the project folder name and the file name for the project files. The
specification is as follows: When a button is pressed, the system generates an 8-bit binary
count, starting at 0, on a bar graph display. The output frequency at the least significant
bit is 50Hz, giving an overall cycle time of 2.56sec.
This specification could be elaborated by, for example, requiring a battery supply. In that
case, an LCD display would be preferred for its low-power consumption over the LED
display used in the prototype.
A block diagram is useful for clarifying the hardware design. The function of each main
circuit block should be identified, as well as the signals in and out. In digital circuits, the
polarity of the signal can be indicated (!Run ⫽ active low input) and a parallel output
represented with a block arrow (x8 ⫽ 8 bits). The standard word processor has all the
drawing tools needed to create simple block diagrams.
Schematic Circuit
The circuit in Figure A.2 shows a PIC 16F877A with crystal clock, push-button input,
and 8-bit bar graph display. The output increments when the button is “pressed” using the
mouse pointer, and the effect can be seen on screen in real time.
The design of the circuit obviously requires knowledge of the relevant interfacing
techniques to connect up peripheral components correctly. For example, the resistor value
in the switch pull-up circuit is not critical, but the maximum value is limited by the input
current drawn by the PIC input; a maximum of 1MΩ is appropriate. At the low end of the
viable range, power conservation is the relevant factor. To limit the current when the switch
is closed, a resistor value of at least 1kΩ is required; 10kΩ is a suitable compromise.
The resistance of each element in the series resistor pack controlling the LED segment
currents must be calculated. If the LED current required is assumed to be 10mA and the
Button Bar Graph
MCU
4MHz
!Run x8
Figure A.1: BAR1 System Block Diagram
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forward volt drop of the LED is 2 V, then the resistor value is given by R ⫽ (5 ⫺ 2)/
(10 ⫻ 10⫺3
) ⫽ 300Ω (NPV ⫽ 270 R).
Refer to Interfacing PIC Microcontrollers (Elsevier, 2005) by the author, for further
information on interface design.
Schematic Edit
ISIS is opened as a discrete package within the Proteus VSM suite. Create a new design
file and save it as BAR1.DSN in a project folder called BAR1, which is accessible from
Proteus and MPLAB.
To start the schematic, the Component button should be clicked to enable the
Devices mode in the object window. The Pick Devices button [P] at the head of the
Object Selector panel gives access to the device libraries (Figure A.3). The category
Microprocessor ICs has a subcategory, PIC 16 Family, from which the PIC 16F877 can
be selected; it then appears in the device list.
Figure A.2: ISIS Schematic Capture Screen
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The bar graph component is picked from the Optoelectronics category, the crystal from
Miscellaneous, and the push button from the Switches and Relays. The resistor and
capacitor are the generic type. ACTIVE components with an associated SPICE model
must be used for interactive testing. Not all components are active, just a representative
selection.
After selection from the object list, a component can be placed with a left click on the
schematic, highlighted (red) with a right click, and removed by right clicking again.
Components are connected together by clicking on the pins in the Component mode.
Wires can be connected, but space on the connecting wire must be allowed between pins.
Always connect in line with a pin and check that a dot appears to confirm that a junction
between pins has been created. The Terminal button brings up the TERMINAL list.
The Ground and Power pins can then be placed. The Power pin automatically adopts the
Vdd of the MCU (⫹5V).
The Overview window allows the schematic to be recentered and displays the
components. The schematic can also be zoomed and centered using the mouse wheel.
Figure A.3: Picking the Microcontroller from the Parts Library
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Components can be oriented or flipped using the rotation and reflection buttons, and
groups of selected components moved or copied using the Tagged Object edit buttons.
Each editing feature should be explored by reference to the Proteus help files and
practical experiment.
The clock circuit and power supplies are implicit in the microcontroller model, so it is not
actually necessary to include the external clock components at this stage. However, they
must be added before a circuit layout is generated in ARES. The simulation clock rate for
the MCU should be set in the component properties dialog when the COF file is attached;
4MHz is usually used in the demo circuits, giving an instruction cycle time of 1μs. This
determines the programmed delay count required to give the specified output rate. If the
output LSB frequency is 50Hz, the period is 20ms. The half-cycle time then is 10ms,
which is the required program delay.
Appendix B explains the program design process in more detail.
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Software Design Using CCS C
A program is to be designed to meet the specification given in Appendix A, which
describes how to develop the hardware design for this application. The specification was as
follows: When a button is pressed, the system generates an 8-bit binary count, starting at
0, on a bar graph display. The output frequency at the least significant bit is 50Hz, giving
an overall cycle time of 2.56sec.
The general form of a real-time application is represented by the flowchart in Figure B.1,
which shows two main phases: initialization and main loop. The initialization is executed
once, and the main loop repeats.
The program must be written to the syntax requirements of standard C, with reference in
this case to the CCS C User Manual (Version 4), downloadable from as a PDF from www.
ccsinfo.com. The dialect of C developed by CCS Inc. is tailored specifically to the features
of the PIC microcontroller. CCS supplies different complier variants for low-, middle-, and
high-performance PICs; the mid-range compiler PCM is used for the PIC 16F877A.
A P P E N D I X B
Initialization
Reset
Control
Loop
Figure B.1: Real-Time Application Flowchart
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The initialization phase typically contains statements that include the MCU-specific
header and library files specific to the target device. The main program is contained
in a function main()
. Variables and data structures defined at this point are global in
scope (recognized and unique throughout the whole program). The endless loop can
be started with while(1) or for(;;), both of which mean to run an endless loop
(unconditionally).
The main loop contains various conditional sequences and loops, comprising data
operations and function calls. These functions may be built into the compiler, included as
additional libraries with the use directive, or written by the user. They process input or
stored data and return results to be used by later functions, for example, as system output.
A general outline of a C program is shown in Listing B.1.
Listing B.1 C Program General Outline
Header comment block
Include resource files
Other preprocessor commands
Function blocks
Function name(plus received parameters)
Local variable data structure declarations
Unconditional sequences
Conditional sequences
Loop sequences
Function calls
Return to calling block with results
Main block
Variable declarations
Data structure definitions
Loop
Unconditional Sequences
Conditional Sequences
Loop Sequences
Function Calls
Endlessly
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BAR1 Source Code
The program source code (Listing B.2) starts with a comment block containing the name
of the project, author, date, version, and program description. Details of the compiler
version, development system, and target hardware can be included. In other words, as
much information as possible to allow the code to be modified, updated, and maintained
effectively. In CCS C, the initialization phase includes a header file that defines the
MCU for which the program is intended. This is necessary as every PIC processor has
Listing B.2 Source Code BAR1.C
/* HEADER COMMENT SECTION ***************************************
BAR1.C MPB V1.0 Source code file details
Output binary count Program description
Simulation version Target system details */
// INITIALIZATION SECTION ***************************************
#include 16F877A.h // Define MCU regsisters etc
#use delay (clock = 4000000) // Include delay routines
void main() // Define main program block
{ // Start of main block
int x; // Declare variable
// CONTROL LOOP SECTION *****************************************
while(1) // Define endless loop
{ // Start of main loop
if(!input(PIN_A4)) // Test input button
{ // Start of conditional block
output_C(x); // Output binary code
x++; // Increment output variable
} // End of conditional block
delay_ms(10); // Wait 10ms
} // End of main loop
} // End of program
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a different set of features: the number of ports, memory size, special input and output
facilities, and so on. The include statement is defined as a compiler (preprocessor)
directive by the leading hash symbol (#). The include directive inserts the source code
from the specified file as though it had been typed in. Your own files can be included, so
you can make a library of your own routines for reuse as required.
Many built-in functions are included by the compiler automatically, for example,
output_C(x). Others have to be specified with use, which identifies a library of
functions used later in the program. The directive #uses delay (clock=4000000)
calls up the set of delay routines that need the MCU clock speed to be stated so that the
correct delays can calculated. The compiler manual indicates which functions need to be
preceded by a use directive.
The initialization phase includes defining all global variables. The variable labels, such as
x or input_value, are attached to the address where the variable value is to be stored.
The variable type declaration (e.g., int) allows the compiler to allocate an appropriate
set of locations for the variable. In CCS C, the default integer size is 8 bits, in others it is
16. Global variables remain in existence while the program is running and are recognized
throughout all levels of the program.
However, to save data memory and allow some duplication of labels, local variables
may be defined within a function. These then exist only for the duration of the function
execution and are subsequently lost. The value of local variables can be passed back to
the calling function or should be defined as global, so that the data are not lost when the
function completes.
PIC Registers
Some knowledge of the PIC internal architecture is useful at this point. The MCU
operation is controlled by a set of file registers, which contain special function registers
(SFRs) in the first 32 locations, followed by some general purpose registers (GPRs).
The 16F877 has four banks of 128 registers, as shown in Figure B.2. Some registers are
duplicated in more than one bank, so the actual number of distinct GPRs is 192.
Figure B.3 shows the function of each bit of the SFRs in Bank0 and Figure B.4 the details
for the status register, which contains the bank select bits. Note that the file register
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Indirect addr. (°)
TMR0
PCL
STATUS
FSR
PORTA
PORTB
PORTC
PORTD(1)
PORTE(1)
PCLATH
INTCON
PIR1
PIR2
TMR1L
TMR1H
T1CON
TMR2
T2CON
SSPBUF
SSPCON
CCPR1L
CCPR1H
CCP1CON
RCSTA
TXREG
RCREG
CCPR2L
CCPR2H
CCP2CON
ADRESH
ADCON0
General
Purpose
Register
96 Bytes
Bank 0
00h
File
Address
01h
02h
03h
04h
05h
06h
07h
08h
09h
0Ah
0Bh
0Ch
0Dh
0Eh
0Fh
10h
1Eh
1Fh
20h
7Fh
11h
12h
13h
14h
15h
16h
17h
18h
19h
1Ah
1Bh
1Ch
1Dh
Indirect addr. (°)
OPTION_REG
PCL
STATUS
FSR
TRISA
TRISB
TRISC
TRISD(1)
TRISE(1)
PCLATH
INTCON
PIE1
PIE2
PCON
SSPCON2
PR2
SSPADD
SSPSTAT
TXSTA
SPBRG
CMCON
CVRCON
ADRESL
accesses
70h–7Fh
ADCON1
General
Purpose
Register
80 Bytes
Bank 1
80h
File
Address
81h
82h
83h
84h
85h
86h
87h
88h
89h
8Ah
8Bh
8Ch
8Dh
8Eh
8Fh
90h
9Eh
9Fh
A0h
EFh
F0h
FFh
91h
92h
93h
94h
95h
96h
97h
98h
99h
9Ah
9Bh
9Ch
9Dh
Indirect addr. (°)
TMR0
PCL
STATUS
FSR
PORTB
PCLATH
INTCON
EEDATA
EEADR
EEDATH
EEADRH
accesses
70h–7Fh
General
Purpose
Register
16 Bytes
General
Purpose
Register
80 Bytes
Bank 2
100h
File
Address
101h
102h
103h
104h
105h
106h
107h
108h
109h
10Ah
10Bh
10Ch
10Dh
10Eh
10Fh
110h
11E h
11F h
120h
16Fh
170h
17Fh
111h
112h
113h
114h
115h
116h
117h
118h
119h
11A h
11B h
11Ch
11Dh
Indirect addr. (°)
OPTION_REG
PCL
STATUS
FSR
TRISB
PCLATH
INTCON
EECON1
EECON2
Reserved(2)
Reserved(2)
accesses
70h–7Fh
General
Purpose
Register
16 Bytes
General
Purpose
Register
80 Bytes
Bank 3
180h
File
Address
181h
182h
183h
184h
185h
186h
187h
188h
189h
18Ah
18Bh
18Ch
18Dh
18Eh
18Fh
190h
19Eh
19Fh
1A0h
1EFh
1F0h
1FFh
191h
192h
193h
194h
195h
196h
197h
198h
199h
19Ah
19Bh
19Ch
19Dh
Figure B.2: PIC 16F877 File Registers (by permission of
Microchip Technology Inc.)
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bank select bits RP0 and RP1 are used for direct addressing, but IRP is used for indirect
addressing via the file select register (FSR).
In this case, the value in the register specified in the FSR is read or written at file address
00. The PIC internal architecture and register operations are fully explained in the
16F87XA data sheet downloadable from www.microchip.com.
Address
Bank 0
00h
(3)
INDF Addressing this location uses contents of FSR to address data memory (not a physical register)
Timer0 Module Register
Program Counter (PC) Least Significant Byte
Indirect Data memory Address Pointer
RE2 RE1 RE0
Write Buffer for the upper 5 bits of the Program Counter
GIE PEIE
ADIF
CMIF
Holding Register for the Least Significant Byte of the 16-bit TMR1 Register
Holding Register for the Most Significant Byte of the 16-bit TMR1 Register
EEIF BCLIF CCP2IF
TMR0IE
T1CKPS1 T1CKPS0 T1OSCEN T1SYNC TMR1CS
TMR2ON
TOUTPS0
TOUTPS1
TOUTPS2
TOUTPS3
Timer2 Module Register
Synchronous Serial Port Receive Buffer/Transmit Register
WCOL SSPOV
Capture/Compare/PWM Register 1 (LSB)
Capture/Compare/PWM Register 1 (MSB)
CCP1X
SPEN
ADCS1 ADCS0 CHS2 CHS1 CHS0 GO/DONE ADON
Legend: x ⫽ unknown, u ⫽ unchanged, q ⫽ value depends on condition, – ⫽ unimplemented, read as ‘0’, r ⫽ reserved.
Shaded locations are unimplemented, read as ‘0’.
RX9 SREN CREN
USART Transmit Data Register
USART Receive Data Register
Capture/Compare/PWM Register 2 (LSB)
Capture/Compare/PWM Register 2 (MSB)
A/D Result Register High Byte
ADDEN FERR OERR RX9D
CCP1Y CCP1M3 CCP1M2 CCP1M1 CCP1M0
CCP2X CCP2Y CCP2M3 CCP2M2 CCP2M1 CCP2M0
SSPEN CKP SSPM3 SSPM2 SSPM1 SSPM0
TMR1ON
T2CKPS0
T2CKPS1
RCIF TXIF SSPIF CCP1IF
TMR0IF
TMR2IF TMR1IF
INTE INTF
RBIE RBIF
PORTB Data Latch when written: PORTB pins when read
PORTA Data Latch when written: PORTA pins when read
PORTC Data Latch when written: PORTC pins when read
PORTD Data Latch when written: PORTD pins when read
IRP RP1 RP0 TO PD Z DC
0000 0000 31, 150
55, 150
30, 150
22, 150
31, 150
43, 150
45, 150
47, 150
48, 150
49, 150
30, 150
24, 150
26, 150
28, 150
60, 150
60, 150
57, 150
62, 150
63, 150
63, 150
64, 150
64, 150
112, 150
133, 150
127, 150
118, 150
118, 150
63, 150
63, 150
61, 150
79, 150
82, 82,
150
0000 0000
0001 1xxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
xxxx xxxx
---- -xxx
---0 0000
-0-0 0--0
--00 0000
--00 0000
--00 0000
0000 00-0
0000 000x
0000 000x
0000 0000
0000 0000
-000 0000
0000 0000
0000 0000
0000 0000
--0x 0000
C
TMR0
PCL
STATUS
FSR
PORTA
PORTB
PORTC
PORTD
PORTE
PCLATH
INTCON
PIR1
PIR2
TMR1L
TMR1H
T1CON
T2CON
SSPBUF
SSPCON
CCPR1L
CCPR1H
CCP1CON
RCSTA
TXREG
RCREG
CCPR2L
CCPR2H
CCP2CON
ADRESH
ADCON0
TMR2
02h
(3)
03h
(3)
04h
(3)
08h
(4)
09h
(4)
0Ah
(1,3)
0Bh
(3)
PSPIF
(3)
0Ch
0Dh
0Eh
0Fh
10h
11h
12h
13h
14h
15h
16h
17h
18h
19h
1Ah
1Bh
1Ch
1Dh
1Eh
1Fh
05h
06h
07h
01h
Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on:
POR, BOR
Details
on page:
Figure B.3: PIC 16F877 Registers, Bank 0 (by permission of
Microchip Technology Inc.)
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BAR1 List File
The list file BAR1.LST, in Listing B.3, shows the assembly language version of the
program produced by the compiler. This book does not assume knowledge of assembler
programming, but for those readers who have followed the usual progression from
assembler, the list file gives a useful insight into how the compiler works. Comments
(italics) have been added to the original file to explain its operation. The original source
code is highlighted in bold.
The compiler initially sets the memory page to 0 by loading the PCLATH (program
counter latch high) register (0A) with 0. This is the reset default setting anyway, but
IRP
bit 7 bit 0
RP1 RP0 TO PD Z DC C
R/W-X
R/W-X
R/W-X
R/W-0
R/W-0
R/W-0 R-1
R-1
bit 7
bit 6–5
bit 4
bit 3
bit 2
bit 1
bit 0
IRP: Register Bank Select bit (used for indirect addressing)
1 ⫽ Bank 2, 3 (100h-1FFh)
0 ⫽ Bank 0, 1 (00h-FFh)
RP1:RP0: Register Bank Select bits (used for direct addressing)
11 ⫽ Bank 3 (180h-1FFh)
10 ⫽ Bank 2 (100h-17Fh)
01 ⫽ Bank 1 (80h-FFh)
00 ⫽ Bank 0 (00h-7Fh)
Each bank is 128 bytes.
TO: Time-out bit
1 ⫽ After power-up, CLRWDT instruction or SLEEP instruction
0 ⫽ A WDT time-out occurred
PD: Power-down bit
1 ⫽ After power-up or by the CLRWDT instruction
0 ⫽ By execution of the SLEEP instruction
Z: Zero bit
1 ⫽ The result of an arithmetic or logic operation is zero
0 ⫽ The result of an arithmetic or logic operation is not zero
DC: Digit carry/borrow bit (ADDWF, ADDLW, SUBLW, SUBWF instructions)
(for borrow, the polarity is reversed)
1 ⫽ A carry-out from the 4th low order bit of the result occurred
0 ⫽ No carry-out from the 4th low order bit of the result
C: Carry/borrow bit (ADDWF, ADDLW, SUBLW, SUBWF instructions)
1 ⫽ A carry-out from the Most Significant bit of the result occurred
0 ⫽ No carry-out from the Most Significant bit of the result occurred
Note: For borrow, the polarity is reversed. A subtraction is executed by adding the two’s
complement of the second operand. For rotate (RRF, RLF) instructions, this bit is
loaded with either the high, or low order bit of the source register.
Figure B.4: PIC 16F877 Status Register Bit Functions (by permission of
Microchip Technology Inc.)
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220. 216 Appendix B
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Listing B.3 List File BAR1.LST
CCS PCM C Compiler, Version 4.024, 37533 16-Feb-07 17:05
Filename: bar1.lst
ROM used: 59 words (1%)
Largest free fragment is 2048
RAM used: 8 (2%) at main() level
9 (2%) worst case
Stack: 1 locations
*; START OF INITIALISATION ****************************
0000: MOVLW 00
0001: MOVWF 0A ; Select Program Page 0
0002: GOTO 01B ; Jump to main block
0003: NOP
.................... /* BAR1.C MPB V1.0
.................... Output binary count
.................... when button pressed
.................... LSB = 50Hz
.................... Simulation version
.................... */
....................
.................... #include 16F877A.h
.................... //////// Standard Header file for the PIC16F877A
device ////////////////
.................... #device PIC16F877A
.................... #list
....................
.................... ; FUNCTION ROUTINE *********************
....................
.................... #use delay (clock = 4000000)
0004: MOVLW 22
0005: MOVWF 04 ; Point to delay value
0006: BCF 03.7 ; Select File Bank 0,1 indirect addressing
0007: MOVF 00,W ; Fetch delay value
0008: BTFSC 03.2 ; If delay value = 0...
0009: GOTO 018 ; ...skip this routine
000A: MOVLW 01 ; START 1ms DELAY LOOP
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000B: MOVWF 78 ; Load delay value high byte = 01
000C: CLRF 77 ; Load delay value low byte = 00
000D: DECFSZ 77,F ; Decrement low counter...
000E: GOTO 00D ; ...and repeat x255 = 765us
000F: DECFSZ 78,F ; Decrement high counter...
0010: GOTO 00C ; ...and do not repeat
0011: MOVLW 4A ; Load low counter...
0012: MOVWF 77 ; ...with 0x4A(74)
0013: DECFSZ 77,F ; Decrement low counter...
0014: GOTO 013 ; ...and repeat x73 = 219us
0015: GOTO 016 ; Next step
0016: DECFSZ 00,F ; Decrement delay value...
0017: GOTO 00A ; ...and repeat 1ms delay loop x9
0018: BCF 0A.3 ; Select program memory page zero
0019: BCF 0A.4
001A: GOTO 039 (RETURN) ; Jump back to main block
....................
....................
.................... ; START OF MAIN BLOCK ***************
.................... void main()
.................... {
001B: CLRF 04 ; Set FSR pointer = 0
001C: BCF 03.7 ; Select File Bank 0,1 for indirect
addressing
001D: MOVLW 1F
001E: ANDWF 03,F ; Select File Bank 0 for direct
addressing
001F: BSF 03.5 ; Select File Bank 1
0020: BSF 1F.0 ; Select analogue input mode 8
0021: BSF 1F.1
0022: BSF 1F.2
0023: BCF 1F.3
0024: MOVLW 07 ; Switch off comparator inputs
0025: MOVWF 1C
.................... int x; ; File register 0x21 (GPR1)
appointed as x
....................
....................
.................... ; START OF MAIN LOOP
....................
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222. 218 Appendix B
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.................... while(1) ; place GOTO 0x29 at main loop end
.................... {
.................... if(!input(PIN_A4))
*
0029: BSF 03.5 ; Select file bank 1
002A: BSF 05.4 ; Set RA4 as input
002B: BCF 03.5 ; Select file bank 0
002C: BTFSC 05.4 ; Test input RA4...
002D: GOTO 036 ; and skip next block if high
.................... {
.................... output_C(x);
*
0026: MOVLW FF
0027: BCF 03.5 ; Select file bank 0
0028: MOVWF 20 ; GPR0 = 0xFF
*
002E: MOVLW 00 ; GPR0 = 0 x 00
002F: MOVWF 20
0030: BSF 03.5 ; Select file bank 1
0031: CLRF 07 ; Port C = output
0032: BCF 03.5 ; Select file bank 0
0033: MOVF 21,W ; Output x
0034: MOVWF 07
.................... x++;
0035: INCF 21,F ; Increment x
.................... }
.................... delay_ms(10);
0036: MOVLW 0A ; Load delay value...
0037: MOVWF 22 ; into GPR2
0038: GOTO 004 ; Jump to delay routine
.................... }
0039: GOTO 029 ; Jump back to start of main loop
....................
.................... }
003A: SLEEP ; Shut down (not normally executed)
Configuration Fuses:
Word 1: 3F73 RC NOWDT PUT NODEBUG NOPROTECT BROWNOUT NOLVP NOCPD NOWRT
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the compiler does not rely on this. The format of the file registers in Bank 0 is shown in
Figure B.2. The program then jumps over the delay function block.
The main block starts by initializing the memory bank selection and the analog inputs.
The variable x is then declared and the compiler allocates file register 0x21 (GPR1)
as its storage location. The statement while(1) at the start of the main loop instructs
the compiler to place a GOTO at the end of the loop with the address of the first loop
instruction as its destination (address 0x29).
The if() statement is implemented by first setting the pin RA4 as input then testing it.
We can see here that the pin initialization is repeated every time the statement is executed.
This is an example of an operation where C is clearly less efficient than assembler,
where the pin would normally be initialized once only. The same problem occurs in the
next block, when the value of x is output—the initialization is repeated each time the
statement is executed.
The delay period (10) is stored in the next available location, 0x22, when the delay is
called. The program then jumps back to the delay code block starting address 0x04. A
counting sequence follows, which gives a delay of 1ms. This is repeated ten times, and
the program jumps back to the main block and the main loop repeats. Note that assembler
instructions CALL and RETURN are not used, because this would limit the number of
nested routines to eight, the limit of the PIC stack depth. By using GOTO instead, this
limitation is avoided by the CCS complier.
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System Testing Using
Proteus VSM
A hardware design schematic BAR1.DSN has been devised (Appendix A) and an
application program BAR1.C developed (Appendix B) from the specification. These can
now be brought together for testing in simulation mode.
Attaching the Program
The application program is output by the C compiler as a file called BAR1.COF, which
should be stored in the project directory BAR1. It contains the machine code plus some
debugging information required by the simulator to display the program source code.
Several other files are created by the compiler at the same time, and all these should be
stored in the same project folder containing the ISIS design file BAR1.DSN.
On the schematic, right click, then left click on the PIC chip to display the component
properties (Figure C.1). The folder browse button allows the COF file to be opened
(attached) to the virtual processor, and the MCU clock frequency can be set at the same
time. The 4 MHz is a useful default clock frequency, as this gives a 1-μs instruction
cycle time and is the maximum frequency using a standard crystal (XT mode in the chip
configuration settings). This clock setting must be passed to the delay routine in the
program. The Program Configuration Word has no significant effect at this stage but must
be set as appropriate when programming real hardware.
Program Debugging
The program can be run by pressing the Play button in the control console. If the program
is correct, the specified output is seen. The bar graph displays a binary count when
A P P E N D I X C
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the Input button on the schematic is “pressed” using the mouse pointer. It should take
2.56 sec to cycle through all the output codes with a loop delay of 10 ms. This can be
checked using the simulation clock at the bottom of the screen.
If the program does not work as required, it needs debugging. The screenshot in
Figure C.2 shows some of the debugging features. The principal technique is single
stepping—the program sequence is checked by executing one statement at a time. This
requires the source code to be displayed; pause the program and select the Debug menu,
PIC CPU Source Code. The source code window appears, with the current execution
point highlighted. If the Pause control is pressed instead of Run, the program can be
single stepped from the first statement. This is useful if the initialization sequence needs
to be checked.
It is not possible to operate the debugging tools and the interactive push button with the
mouse at the same time. Therefore, in Figure C.2, the Input button is shorted out with a
temporary link so that the output runs continuously. Alternatively, it can be replaced with
a switch for simulation purposes.
The source code window has a selection of debug buttons: Run, Step Over, Step Into, Step
Out Of . . . the current function. Step Over means execute the following function call at
full speed, stopping on return, while Step Into means execute the function stepwise. While
stepping through a function, Step Out Of allows you to return to the calling block at full
speed. This is useful for getting out of a function you have inadvertently stepped into.
Figure C.1: MCU Properties Dialog for Attaching the Program
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The Breakpoint button is used to set and clear breakpoints in the code at the current
cursor position. Program execution is run at full speed, until stopped at the breakpoint.
Additional breakpoint control options can be selected by right clicking on the source code
window. This source window menu also allows the display to be modified to show line
numbers and program memory addresses. The assembler code for each statement can be
displayed by selecting Disassembly. Note that several lines of assembler code are needed
for each C statement—this is the reason that the C program needs more memory. The Set
Font option is useful if displaying the PC screen on a projector (teachers note); the text
can be enlarged for better visibility.
PIC CPU variables are displayed from the Debug menu. Right click on the window
and deselect the Globals option, leaving just the program variables visible. The display
numerical format can then be changed by right clicking on the variable in the window, for
example, to display the variables as unsigned integers if only positive whole numbers
are used.
The CPU registers may be displayed if required, as well as the CPU data memory, that
is, the file registers. Some of these have special or system functions, the rest are available
for variable storage. Remember that some variable types use more than one location;
Figure C.2: Program Simulation Screenshot
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227. 224 Appendix C
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for example, a 16-bit integer uses two. The variable locations are highlighted when they
change during single stepping.
If you need to slow down the program execution, go to the System, Animation options.
The Frames per Second and Timestep per Frame settings control the simulation speed.
The default settings are 20f/s and 50ms/f, giving 2050 1000ms/sec, or real time. If
the Timestep per Frame is reduced to, say, 5ms, the simulation slows down by a factor
of 10. This allows the system operation to be observed at a more leisurely pace in Run
mode. In complex applications, the simulation may slow down automatically to allow the
processor to complete the circuit solution for each simulation step, in which case, it does
not run in real time. This can be checked by observing the simulation clock display.
Typical Errors
The types of errors that appear when the program is compiled are either syntax or linker
errors. A syntax error might be a spelling mistake in the source code or an undeclared
variable. Linker errors appear when the program files are combined to create the final
program; a common one is that the include files have not been placed in the project
folder and cannot be found by the linker.
Logical errors, on the other hand, appear only when the program is tested; and these are
easier to correct if detected prior to downloading to hardware, by using a simulator such
as MPSIM or VSM. VSM is easier to use, as the errors are more readily spotted in the
animated schematic than in the numerical output of MPSIM.
Some simple examples of possible errors in BIN1.C are outlined next.
Sequence Error
While the increment statement follows the output statement, the first output is 00000000.
If, instead, the increment were placed before the output, the first output is 00000001,
and this is not as specified. This error is not evident in the Run mode but is detected if the
program is single stepped from the top (hit Pause initially rather than Run).
Inversion Error
This is a logical error that causes the opposite effect to that required. For example, if the
exclamation mark is omitted before the input function, the output runs when the button is
open rather than closed.
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Parameter Error
If the wrong input is specified in the input statement (e.g., PIN_A5 instead of PIN_A4),
the button has no effect, as the wrong input is being tested. This error is detected by
comparing the program and schematic.
Timing Error
The delay time is calculated so that the LSB toggles every 10ms. If this figure is
incorrect, the output frequencies are wrong. This can be checked by using the
simulation clock or a virtual oscilloscope.
The simulation clock is displayed at the bottom of the schematic window. To check
the period of the output, a breakpoint can be set at the beginning of the main loop. The
program then stops once per cycle, and the time taken per cycle can be read from the
clock. A breakpoint is set by clicking on the Breakpoint button at the top of the source
code widow.
The oscilloscope allows the output to be displayed in the time domain. It is selected
from the Virtual Instruments list. Input A should be connected to the output, RC0, and
Figure C.3: Virtual Oscilloscope Screenshot
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a full-size version of the scope should appear when the simulation is run. If not, enable it
in the Debug menu. Adjust the controls to see the 50-Hz waveform displayed.
Figure C.3 shows the VSM analog scope and simulation clock display. A breakpoint has
been set at the if statement, so the clock increments by 10ms each time Run is selected.
ISIS also provides virtual signal sources, meters, voltage and current probes, logic
analyzer, and counter/timer, as well as a graphing feature for analog and digital signals.
When the program is fully debugged, it can be downloaded to hardware and retested. This
should leave only hardware faults to be rectified to obtain a working system.
Readers should note that Proteus VSM is continuously updated. New features and
components are added on a regular basis. Specifically, new MCUs are added as they are
released by the manufacturers. Version 6 was used to produce the simulation circuits in
this book. Version 7 has since been released, which has, for example, an enhanced
4-channel virtual oscilloscope. Visit www.labcenter.co.uk for the latest product
information.
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C Compiler Comparison
The intention of this book is to introduce C programming for all microcontrollers.
However, particular products have to be selected to act as examples. When the
basics have been explained using one particular combination of MCU, compiler, and
development system, others can be considered.
The CCS C compiler was selected for this book principally because it has an extensive
library of peripheral driver routines, is reasonably inexpensive, and is recognized by
Microchip and Labcenter as a preferred compiler. However, several other suitable
compilers are available at the time of this writing, so it would be useful to see how they
compare. The following products have been selected, but bear in mind that, in the rapidly
moving microcontroller market, significant changes probably have occurred by the time
you read this:
● Microchip C18
● HiTech PIC C
● Mikroelektronika C
● Matrix Multimedia C
The first two are professional compilers, which would tend to be used by more
experienced engineers. The second two are aimed at the educational market and include
more user-friendly features to help the beginner.
Other PIC C compilers are available that are not considered here. They are typically
supplied by companies that produce development tools for a range of different processors,
which could suit application developers who use a range of MCU types. They do not
provide the range of library functions considered essential here.
A P P E N D I X D
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Each compiler has a set of header files provided, all of which have a similar function of
defining the register and control bit labels for all the supported processors. The exact
labeling system can vary, although the labeling used in the PIC hardware manuals must
be preferred.
Microchip C18
Microchip does not supply a compiler for the mid-range 16 series MCUs. It is assumed
that any application developed in C will be run on an 18 series processor or above. This
is because the mid-range devices have limited memory capacity, and many commercial C
applications exceed this limit.
Nevertheless, it is well worth looking at C18, because having learned C on the 16 series,
the reader may wish to consider the option of progressing to the 18 series for further
work. The full list of features claimed for this compiler, as listed in the C18 User Guide
(www.microchip.com) includes
● ANSI ‘89 compatibility.
● Integration with the MPLAB IDE for easy-to-use project management and source-
level debugging.
● Generation of relocatable object modules for enhanced code reuse.
● Compatibility with object modules generated by the MPASM assembler, allowing
complete freedom in mixing assembly and C programming in a single project.
● Transparent read/write access to external memory.
● Strong support for in-line assembly when total control is absolutely necessary.
● Efficient code generator engine with multilevel optimization.
● Extensive library support, including PWM, SPI™, I2
C™, UART, USART, string
manipulation, and math libraries.
● Full user-level control over data and code memory allocation.
It must be assumed that the integration of C18 into the MPLAB IDE will be reasonably
seamless, giving it a built-in advantage over competing compilers. Source-level
debugging, in particular, can reveal limitations in the effectiveness of the integration into
the IDE of a third party product.
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Relocatable object modules allow the user to build up a library of reusable routines. This
is obviously useful when producing a series of similar application programs. If particular
hardware peripherals are used repeatedly in different designs, the same driver routines,
perhaps with minor variations, can be used. However, these routines must be designed to
receive and return variable values in a consistent manner to maximize the benefits of this
approach.
Library routines are provided for the main peripheral interfaces, and a comprehensive
selection is found in the C18 Compiler Libraries manual. Software drivers allow
peripherals to be connected to any pin, not just those associated with the internal
hardware interface. This provides more flexibility in the use of the chip pins and may
mean that a cheaper device can be used for a particular application.
If we look at some source code examples provided in the C18 User Guide, we may be
able to identify some of the features where C18 and CCS C diverge. Remember, however,
that the general language syntax must conform to the ANSI standard. Listing D.1 shows
a simple LED flasher program.
Listing D.1 C18 Sample Source Code (LED Flasher)
#include p18cxxx.h /* MCU header file ***********/
void delay (void) /* Delay function *************/
{
unsigned int i;
for (i=0; i10000; i++);
}
void main (void) /* Main Program ***************/
{
TRISB=0; /* Port B output */
while(1) /* Loop always */
{
PORTB=0; /* Reset the LEDs */
delay(); /* Delay to see change */
PORTB = 0x5A; /* Light the LEDs */
delay(); /* Delay to see change */
}
}
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The MCU header file is included in the same way as in CCS C, and the delay routine
uses standard syntax. The main difference evident is that the port registers are addressed
directly by assigning a value to the data direction register (e.g., TRISB=0) and the output
data register (e.g., PORTB=0x5A). In CCS C, a function is used (output_B(0)). The
C18 syntax is arguably simpler.
Listing D.2, a C18 program using interrupts, illustrates some other differences. As in
many PIC C compilers, direct access to the register control bits is used, for example,
in the statement INTCONbits.TMR0IF=0, which resets the timer interrupt flag. This
requires knowledge of the internal architecture, which makes the programming more
difficult. CCS C sensibly avoids the need for such direct access. The timer setup
statement uses a function call in a similar format to CCS, but of course, the exact
syntax is different.
Listing D.2 also includes other features not covered elsewhere in this book. The #pragma
directive allows additional directives to be defined for this specific compiler and added to
the standard set defined in the ANSI standard. The keywords _asm and _endasm enclose
a section of assembly language code, in this case just one instruction GOTO label.
Hi-Tech PIC C
The Hi-Tech PIC C is a professional standard compiler supplied by a company well
established as a development system tool supplier. Hi-Tech supplies C compilers for
wide range of microcontrollers on the market: PIC 16, 18, 24, and dsPIC (digital signal
processors) as well as Freescale 68000-based types, ARM, 8051 derivatives, Texas
Instruments MSP430 devices, and other legacy products.
The features claimed are these:
● ANSI C—full featured and portable.
● Reliable—mature, field-proven technology.
● Multiple C optimization levels.
● An optimizing assembler.
● Full linker, with overlaying of local variables to minimize RAM usage.
● Comprehensive C library with all source code provided.
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● Support for 24-bit and 32-bit IEEE floating point and 32-bit long data types
included.
● Mixed C and assembler programming.
● Unlimited number of source files.
Listing D.2 C18 Sample Source Code (LED Output Using Timer Interrupt)
#include p18cxxx.h
#include timers.h
#define NUMBER_OF_LEDS 8
void timer_isr (void);
static unsigned char s_count=0;
#pragma code low_vector=0x18
void low_interrupt (void)
{
_asm GOTO timer_isr _endasm
}
#pragma code
#pragma interruptlow timer_isr
void timer_isr (void)
{
static unsigned char led_display=0;
INTCONbits.TMR0IF=0;
s_count=s_count % (NUMBER_OF_LEDS+1);
led_display =(1 s_count++)-1;
PORTB=led_display;
}
void main (void)
{
TRISB=0;
PORTB=0;
OpenTimer0 (TIMER_INT_ON T0_SOURCE_INT T0_16BIT);
INTCONbits.GIE=1;
while (1){}
}
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● Listings showing generated assembler.
● Compatible—integrates into the MPLAB®
IDE, MPLAB ICD, and most third
party development tools.
● Runs on multiple platforms: Windows®
, Linux®
, UNIX®
, Mac OS X, Solaris™.
Optimization involves reducing the final code size by removing redundant code and
modifying the assembler version to reduce the number of instructions to the minimum
achievable.
The most obvious disadvantage of this compiler is that only the standard library functions
for data conversion, memory management, mathematical operations, and basic I/O are
provided. It is assumed that the user will develop the peripheral drivers as required, to suit
the particular range of applications and hardware to be supported, or that the peripheral
control registers will be accessed directly.
On the other hand, a major advantage is that a fully featured freeware version, PICC-
Lite, is available for hobbyists, students, and limited commercial purposes. At the
time of writing, the following PIC MCUs are supported with no limitations, as
compared to the full version: 12F629, 12F675, and 16F84. A further set of 16 series chips
can be used with a limitation on RAM and program memory: ‘627, ‘684, ‘690, ‘877,
‘887, and ‘917. Other limitations are imposed due to the limited memory available in
these chips.
Hi-Tech also supplies Salvo RTOS, including a freeware version. This is a cooperative,
event-driven, priority-based, multitasking, real-time operating system designed for
microcontrollers with limited RAM and ROM. The manual supplied (www.pumpkininc.
com) with this product contains a very useful introduction to RTOS principles and is
recommended if further information is required on using RTOS in PICs.
An example of Hi-Tech C source code is shown in Listing D.3. It outputs a binary count
at Port B that is incremented every second using a timer interrupt. The port register is
addressed directly, using the label PORTB. The timer control bit labels are defined in the
header file PIC.H and set directly in the main routine. Note that here the calculation of
the initial loop count constant RELOADS is calculated in the initial directive block using
the arithmetic and logic operations provided within the directive syntax. Recall that CCS
C uses a directive to declare a function as an ISR; here, the compiler recognizes the
keyword interrupt within the function name instead.
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Listing D.3 Hi-Tech C Sample Source Code (Timer Interrupt)
#include pic.h
/* Example code for using timer0 on a 16F84
Sets up a 1 second interrupt and increments Port B
*/
/* Calculate preload value for one second timer ************/
#define PERIOD 1000000 // Period in us-one second here
#define XTAL 4000000 // Crystal frequency-4MHz
#define IPERIOD (4 * 1000000/XTAL) // Period of instruction clock in us
#define SCALE 256 // Timer 0 prescaler
#define T0_TICKS 256 // Number of counts for interrupt
#define TICK_PERIOD (SCALE * IPERIOD) // Period (us) of timer clock
#define RELOADS((PERIOD/T0_TICKS)/ // Calculate preload value
TICK_PERIOD)
unsigned long seconds; // Second count
near char reload = 0; // Reload count
/* Service routine for timer 0 interrupt *******************/
void interrupt timer0_isr(void) // Define function as timer ISR
{
if(reload == 0){
reload = RELOADS + 1; // Set initial value of reload
count
seconds++; // Count seconds
PORTB++; // Change port display
}
reload--; // Count down reloads
T0IF = 0; // Clear timer interrupt flag
}
main() /* Initialise timer and wait for
interrupt *************/
{
OPTION = 0b0111; // prescale by 256
T0CS = 0; // select internal clock
T0IE = 1; // enable timer interrupt
GIE = 1; // enable global interrupts
TRISB = 0; // output changes on LED
for(;;)
continue; // let interrupt do its job
}
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Mikro C
Mikroelectronica supplies range evaluation and development boards for the PIC and other
microcontrollers, as well as C, Pascal, and Basic compilers (Figure D.1). The C compiler
MikroC is well documented in a downloadable user manual and includes a good range of
peripheral driver libraries, including CAN, Ethernet, and graphical LCD drivers as part
of a comprehensive I/O library. The packages are oriented toward the educational and
hobby market, offering additional features designed to assist the beginner in developing C
applications.
An evaluation version does not appear to be available at the time of this writing, and the
compiler syntax can be assessed prior to purchase only by reference to code fragments
given in the manual. An ADC input block is reproduced as an example in Listing D.4. As
we see, the control registers are set up by loading control codes as hex numbers, which
requires the program designer to look up the necessary bit configurations. However, the
ADC access function is simple and concise, allowing the input channel to be selected as
the function parameter.
Matrix C
The primary product line of Matrix Multimedia is a user-friendly hardware system,
E-blocks, that allows different systems to be assembled using plug-in modules. The
Figure D.1: Mikroelectronica EasyPIC4 Development Board
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processor module incorporates sockets for a range of PIC MCUs and a number of
D-type connectors. Peripheral modules with push buttons, LEDs, displays, keypad,
relays, communications interfaces, and so on are added as required (Figure D.2).
Listing D.4 MikroC Source Code Sample (ADC Input and Display)
unsigned inval; // 16-bit integer for 10-bit input
void main {
ADCON1 = 0x80; // Setup ADC
TRISA = 0xFF; // Analog inputs
TRISB = 0x3F; // RB6,RB7 display outputs
TRISD = 0; // Port D display outputs
do{
inval = Read_ADC(2); // Read channel 2 (RA2)
PORTD = inval; // Show low 8 bits
PORTB = inval2; // Show high 2 bits
}while(1);
}
Figure D.2: Matrix Multimedia Modular PIC System
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Listing D.5 Matrix C Source Code Sample (ADC Input and Display)
#include system.h
void setupADC(void)
{ trisb = 0x00; /* Port B display */
trisa = 0xf1; /* RA0 input, RA1-3 output */
adcon0 = 0x00; /* Set up ADC */
adcon1 = 0x80; /* Set up ADC */
ansel = 0x01; /* Select AN0 only */
}
void main(void)
{
setupADC(); /* Call setup function */
while (1) /* Loop always */
{ adcon0 = 0x05; /* Start ADC */
while(adcon00x04); /* Wait until done */
portb = adresl; /* Display low byte */
porta = adresh*2; /* Display high bits */
}
}
The application programming can be implemented using a choice of assembler or C.
Matrix also offers a proprietary flowchart-based programming system, Flowcode. The
program is constructed using flowchart blocks, which are automatically converted to
C and hence to assembler and machine code.
The C syntax used is illustrated in Listing D.5—a simple program to read an analog
input and display the result. As in many C compilers for PIC, the control registers are
loaded directly, and no special functions are used for peripheral access. The programming
system is described via a tutorial, which is included with the compiler, so no separate
reference manual is provided.
Summary of C Compilers
The features of the C compilers for the PIC 16 series MCUs outlined in this appendix are
compared in Table D.1. We are particularly interested in using the 16F877, our reference
device, which is used in the demo applications in the main part of this book. The
compilers have been divided into commercial and educational categories.
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Microchip C18 and Hi-Tech C are designed primarily for professional use, as reflected in
the relatively high price, but this is compensated for by the provision of feature-limited
freeware versions. For any development engineer who will be using mainly PIC 18 or
above parts, the C18 offers the advantage of extensive function libraries. Bear in mind
though, a separate compiler, C30, is needed for PIC24 and dsPIC devices, although one
can assume an easy progression route from C18. For those intending to use a wider range
of MCU types, Hi-Tech might be preferred. Hi-Tech PICC Lite offers good functionality
in a limited range of PIC 16 devices, including 16F877.
Table D.1: Comparison of C Compilers for PIC 16 Series M
Microchip C18 Hi-Tech C CCS C Mikro C MM C
URL (microchip.
com)
(htsoft.com) (ccsinfo.
com)
(mikroe.com) (matrixmultimedia.
com)
Primary
market
Commercial Commercial Both Educational Educational
MCU
targets
PIC 18 only PIC ⫹ others PIC only Mainly PIC Mainly PIC
Primary
target
hardware
Any Any Any Proprietary
single board
Proprietary
modular
Function
libraries
Extensive
peripheral
support
Standard
libraries only
Good
peripheral
support
Extensive
peripheral
support
Standard libraries
only
Tutorial
or user
manual
Comprehensive
free download
Comprehensive
free download
Free
download
Comprehensive
free download
Tutorial in package
only
Relative
price
(single
user)
PIC16 n/a
PIC18 $495
PIC24 $895
PIC16 $995
PIC18 $995
PIC24 $1195
16F87X $50
PIC16 $150
PIC18 $200
PIC24 $250
PIC16⫹18 $249
PIC24 $249
PIC16 $99*
PIC16⫹18 $180*
Demo
version
Function-
limited student
edition
Time-limited
evaluation
version
Time- and
memory-
limited demo
None None
Origin US US US EU UK
*Approximate
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The educational compilers are designed primarily as components of training packages
consisting of hardware, development system, compiler, tutorials, proprietary simulation
software, and so on. These products should certainly be considered if a complete
package is required, for example, by a college or university upgrading its resources.
The Mikroelectronika packages are oriented more toward the hobby market, while the
Matrix Multimedia product range is suitable for a wide range of education institutions,
from schools to universities. The support materials provided with the Matrix Multimedia
compiler are very closely tied to the training packages, so no separate compiler manual is
provided, for example. For the hobbyist and independent learner, Mikro C is supported by
a comprehensive and fully documented function library.
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CCS C Programming
Syntax Summary
Compiler Directives
#include source files Include source code or header file
#use functions(parameters) Include library functions
#define oldtext newtext Replace label in source code with value
#device name Identify MCU type
#list, #nolist Turn on source code listing
#asm, #endasm Start/end of assembler block
#fuses options Select MCU configuration fuse settings
#int_xxx Declare function as interrupt service routine
Program Blocks
main(condition) {statements} Main program block
while(condition) {statements} Conditional loop
do{statements} while(condition) Conditional loop
if(condition) {statements} Conditional sequence
for(begin;end;next) {statements} Preset loop conditions
switch(x)..case n: Multichoice selection
Punctuation
/* Comments */ Star/slash enclose block comment
statement; // Comment Double slash before line comment
{ statement; statement; } Braces enclose program block
statement; Semicolon end of statement
funcname(arg1,arg2) Function arguments/parameters, comma separates
[n] Array size, variable
text ASCII function argument/include filename
'y' ASCII value
A P P E N D I X E
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Basic I/O Functions
output_X(n) Output 8-bit code at Port X
output_high(PIN_Xn) Set output bit high
output_low(PIN_Xn) Set output bit low
input(PIN_Xn) Get bit input
n=input_X() Get byte input
Variable Types
Identifier Type Min Max Range
int1 1 bit 0 1 1 20
unsigned int8 8 bits 0 255 256 28
signed int8 8 bits ~127 ~127 255 28
1
unsigned int16 16 bits 0 65,535 65,536 216
signed int16 16 bits ~32,767 ~32,767 65,535 216
1
unsigned int32 32 bits 0 4,294,967,295 4,294,967,296 232
signed int32 32 bits ~2,147,483,647 ~2,147,483,647 4,294,967,295 232
1
float 32 bits ~10–39
~10138
~1077
Relational Operators
Operation Symbol Example
Equal to == if(a == 0) b=b+5;
Not equal to != if(a != 1) b=b+4;
Greater than if(a 2) b=b+3;
Less than if(a 3) b=b+2;
Greater than or equal to = if(a = 4) b=b+1;
Less than or equal to = if(a = 5) b=b+0;
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Formatting Codes
Code Displays
%d Signed integer
%u Unsigned integer
%Lu Long unsigned integer (16 or 32 bits)
%Ls Long signed integer (16 or 32 bits)
%g Rounded decimal float (use decimal formatting)
%f Truncated decimal float (use decimal formatting)
%e Exponential form of float
%w Unsigned integer with decimal point inserted (use decimal formatting)
%X Hexadecimal
%LX Long hex
%c ASCII character corresponding to numerical value
%s Character or string
Arithmetic and Logic Operators
1 Operand Arithmetic, 2 Operands Logic, 2 Operands
Assign value, = Add, + AND,
Increment, ++ Subtract, - OR, |
Decrement, -- Multiply, * XOR, ^
Complement, ~ Divide, /
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CCS C Program Function
Reference
This is a summary of the more commonly used functions available in CCS C Version
4 (January 2007). For more details on how to use the listed functions and others not
included here, visit www.ccsinfo.com for a current manual download.
The following apply to all the following tables:
1. All functions require a header file, e.g., 16F877A.H.
2. The numerous CAN and USB functions are not included since these interfaces
are not typically available in 16 series MCUs.
3. Alternative functions for the same operation:
putc() == putchar()
getc() == getch() == getchar()
output_bit(PIN_XX,1) == output_high(PIN_XX)
output_bit(PIN_XX,0) == output_low(PIN_XX)
get_timer0() == get_rtcc();
set_timer0(nnn) == set_rtcc(nnn);
pow() == pwr()
A P P E N D I X F
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Table F.1: Port Input and Output
(Requires Chip Header File Only, e.g., 16F877A.H)
Function Description Example Comment
WRITE BYTE Write all bits with 8-bit
integer
output_A(255); A replaced by B, C, D,
or E
SET BIT Write output bit high
using pin label
output_high
(PIN_A0);
A0 replaced by A1,
A2, . . ., A7, B0, . . .,
B7, etc.
CLEAR BIT Write output bit low
using pin label
output_low
(PIN_A0);
A0 replaced by A1,
A2,. . ., A7, B0, . . .,
B7, etc.
READ BYTE Read input as 8-bit
integer
abyte =
input_A();
A replaced by B, C,
D, or E
READ BIT Read input bit using
pin label
abit =
input(PIN_A0);
A0 replaced by A1,
A2, . . ., A7, B0, . . .,
B7, etc.
READ DIRECTION Check port data
direction register
ddra =
get_tris_a();
Any parallel port ddr
code can be checked
CHECK BIT Read input bit abit =
input_state
(PIN_D0);
Gets I/O bit value
BIT TOGGLE Toggle output bit output_toggle
(PIN_D0);
Invert the logic level at
the specified pin
BIT OUTPUT Change port bit to
output
output_drive
(PIN_D0);
Does not change the
existing bit value
FLOAT OUTPUT Set output pin to high
impedance
output_float
(PIN_D0);
Allows an external
source to control the
line
SET PULLUPS Switch input pull-ups
on or off
port_a_pullups
(TRUE);
Input floats to high
value, port A or B only
SET DIRECTION Initialize port bits for
input or output
set_tris_a
(0x0F);
Explicitly sets up data
direction register
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Table F.2: Analog Inputs (Requires #DEVICE ADC ⴝ nn)
Function Description Example Comment
SETUP Initialize ADC setup_adc(ADC_CLOCK_
INTERNAL);
All modes listed in
device header file
PINS SETUP Initialize ADC pins setup_adc_ports
(RA0_ANALOG);
All modes listed in
device header file
CHANNEL SELECT Select ADC input set_adc_channel(0); Channels 0–7 selected
via multiplexer
READ Read analog input inval = read_adc(); 8-bit read 0–255,
10-bit read 0–1024
(#device option)
Table F.3: Timers (Requires Chip Header File Only, e.g., 16F877A.H)
Function Description Example Comment
TIMERX SETUP Set up the timer mode setup_timer0
(RTCC_INTERNAL
| RTCC_DIV_8 );
Clock source and
prescale ratio
TIMERX READ Read a timer register
(8 or 16 bits)
count0 =
get_timer0();
Timer numbers (0–5)
valid as fitted
TIMERX WRITE Preload a timer register
(8 or 16 bits)
set_timer0(126); Timer numbers (0–5)
valid as fitted
TIMER CCP SETUP Select PWM, Capture,
or Compare mode
setup_ccp1
(ccp_pwm);
See CCS manual for
CCP options
TIMER PWM DUTY Set PWM duty cycle set_pwm1_duty
(512);
512 ⫽ mark
count ⫽ 50%
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Table F.4: RS232 Serial Port
(Requires #USE RS232, #USE DELAYS (Clock⫽nnnnnnnn))
Function Description Example Comment
SET BAUD RATE Set hardware RS232
port baud rate
setup_uart(19200); Applies to hardware
serial port only
SEND BYTE Write a character to the
default port
putc(65) Writes ASCII data or
control code to serial
output
SEND SELECT Write a character to
selected port
s = fputc(A,01); As preceding, but
stream identifier given
PRINT SERIAL Write a mixed message printf(Answer:
%4.3d,n);
Write fixed strings and
formatted variable
values
PRINT SELECT Write string to selected
serial port
fprintf
(01,Message);
As preceding, but
stream identifier given
PRINT STRING Print a string and write
it to array
sprintf
(astr,Ans=%d,n);
Print and copy output
to character array
RECEIVE BYTE Read a character to an
integer
n = getc(); Waits for ASCII code
from serial input
RECEIVE
STRING
Read an input string to
character array
gets(spoint); Reads characters into
an array at address
RECEIVE SELECT Read an input string to
character array
astring =
fgets(spoint,01);
As preceding, but
string and stream
identifier given
CHECK SERIAL Check for serial input
activity
s = kbhit(); Checks for serial input
data but does not wait
PRINT ERROR Write programmed
error message
assert(a3); Generates an error
message if condition is
FALSE
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Table F.5: SPI Serial Port (spi Can Be Replaced by spi2)
Function Description Example Comment
SPI SETUP Initialize SPI serial port setup_spi
(spi_master);
See CCS manual for
full list of options
SPI READ Receives data byte from
SPI port
inbyte =
spi_read();
Waits for 8-bit data to
arrive
SPI WRITE Sends data byte via SPI
port
spi_write
(outbyte);
Writes 8-bit data to
SPI serial line
SPI TRANSFER Send and receive via SPI inbyte =
spi_xfer
(outbyte);
See CCS manual for
variations
SPI RECEIVED Check if SPI data received done = spi_data_
is_in();
Returns 0 for not
done, 1 if done
Table F.6: I2C Serial Port
(#USE I2C() If Hardware Peripheral Fitted, #DEFINE for Software Interface)
Function Description Example Comment
I2
C START Issue start command in
master mode
i2c_start(); Start a data
transmission
I2
C WRITE Send a single byte i2c_write
(outbyte);
Send a data byte
I2
C READ Read a received byte inbyte =
i2c_read();
Read a data byte
I2
C STOP Issue a stop command in
master mode
i2c_stop(); Stop the data
transmission
I2
C POLL Check to see if byte received sbit =
i2c_poll();
Returns 1 if byte
waiting
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Table F.9: Register Manipulation
Function Description Example Comment
REGISTER BIT
SET
Set a selected bit bit_set(num,1); Sets bit b in integer
num (8, 16, or 32 bits)
REGISTER BIT
CLEAR
Clear a selected bit bit_clear(num,2); Clears bit b in integer
num (8, 16, or 32 bits)
REGISTER BIT
TEST
Test a selected bit flag =
bit_test(num,4);
Tests bit b in integer
num (8, 16, or 32 bits)
REGISTER SWAP Swap nibbles in a
byte variable
swap(abyte); Result not returned by
function
Table F.8: LCD Control (Requires Chip Header File Only, e.g., 16F877A.H)
Function Description Example Comment
LCD SETUP Set up LCD internal
control
setup_lcd
(LCD_MUX12,1);
Number of control
lines, clock prescale
LCD LOAD Send display data
block to LCD
lcd_load
(lcddata,0,16);
Pointer, offset, number
of bytes
LCD SYMBOL Send segment bits lcd_symbol
(lcddata,dig1)
Specify segments
individually
Table F.7: Parallel Slave Port
Function Description Example Comment
PSP ENABLE Enable or disable PSP setup_psp
(PSP_ENABLED);
PSP_DISABLED to
switch offSET.
SET DIRECTION Set the PSP data direction set_tris_e(0); For input arg. = 0xFF,
or mixed mode
OUTPUT READY Checks if output byte is
ready to go
pspo = psp_
output_full();
Byte ready: pspo = 1
To write the PSP:
PSP_DATA = outbyte;
INPUT READY Checks if input byte is
ready to read
pspi = psp_
input_full();
Byte ready: pspi = 1
To read the PSP:
inbyte = PSP_DATA;
PSP OVERFLOW Checks for data overwrite
error
pspv = psp_
overflow();
Check to prevent loss
of data due to external
mistiming
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Table F.10: Block Rotate
Function Description Example Comment
BLOCK ROTATE
LEFT
Rotates bits of
structure left
rotate_left
(lobyte,6);
Address of low byte
and number of bytes
BLOCK ROTATE
RIGHT
Rotates bits of
structure right
rotate_right
(lobyte,10);
Address of low byte
and number of bytes
BLOCK SHIFT
LEFT
Shift bit left into low
bit of structure
shift_left
(lobyte,4,1);
Address of low byte,
number of bytes, bit in
BLOCK SHIFT
RIGHT
Shift bit right into high
bit of structure
shift_left
(lobyte,4,1);
Address of low byte,
number of bytes, bit in
Table F.11: Math Functions (#INCLUDE MATH.H)
Function Description Example Comment
ABSOLUTE
VALUE
Absolute value of
integer
abres = abs(x); Returns unsigned
positive value of signed
integer
LONG
ABSOLUTE
Absolute value of
long integer
longres = labs(x); Returns unsigned
positive value of 16-bit
integer
FLOAT
ABSOLUTE
Absolute value of
float
flores = fabs(x); Returns unsigned
positive value of signed
float
FLOAT
CEILING
Round a float up to
integer
roundup =
ceil(afloat);
Returns integer from
float
FLOAT FLOOR Round a float down
to integer
roundown =
floor(afloat);
Returns integer from
float
INTEGER
DIVIDE
Integer divide divres =
div(numer,denom);
Returns a structure
of quotient and
remainder
LONG DIVIDE Long integer divide lonres =
ldiv(lnumer,ldenom);
Returns a structure
of quotient and
remainder
EXPONENTIAL Exponential function expres = exp(x); Returns exp where x is
a float
(continued)
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Table F.11: (continued)
Function Description Example Comment
LOG BASE 10 Logarithm base-10
function
logres == log10(x); Returns log10(x)
where x is a float
LOG BASE E Logarithm base-e
function
lnres = log(x); Returns ln(x) where
x is a float
DIVISION
MODULUS
Modulus (remainder)
of division
modres =
fmod(numer,denom);
Returns remainder of
float division
FRACTION
MODULUS
Break up float into
integer and fraction
modfres =
modf(afloat,whole);
Returns fractional
part, stores integer
FRACTION
EXPAND
Break up float into
integer and fraction
fexres =
frexp(afloat,whole);
Returns fractional part
BINARY
EXPAND
Multiply a float by
integral power of 2
lexres =
ldexp(afloat,sint);
Returns a float, sint
is a signed integer
RAISE TO
POWER
Raise float to a
power
powres =
pow(afloat,apower);
Returns a float raised
to a power
SQUARE ROOT Calculate the square
root of a float
sqrres =
sqrt(afloat);
Returns positive root
RANDOM
NUMBER
Generates a
pseudorandom
number
any1 = rand(); Returns a random
integer from sequence
RANDOM SEED Start value for the
“random” sequence
srand(seed); seed is a new start
point in the sequence
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Table F.12: Trigonometric Functions (#INCLUDE MATH.H)
Function Description Example Comment
SIN Sine function num1 = sin(a); Returns sine of angle a given in
radians
COS Cosine function num2 = cos(a); Returns cosine of angle a given
in radians
TAN Tangent function num3 = tan(a); Returns tangent of angle a given
in radians
ASIN Arc sine function ang1 = asin(n); Returns the angle in radians
whose sine is float n
ACOS Arc cosine function ang2 = acos(n); Returns the angle in radians
whose cosine is float n
ATAN Arc tangent function ang3 = atan(n); Returns the angle in radians
whose tangent is float n
SINH Hyperbolic sine function hyp1 = sinh(x); Returns hyperbolic sine of float x
COSH Hyperbolic cosine
function
hyp2 = cosh(x); Returns hyperbolic cosine of
float x
TANH Hyperbolic tangent
function
hyp3 = tanh(x); Returns hyperbolic tangent of
float x
Table F.13: Make Integers
Function Description Example Comment
MAKE BYTE Extract a byte from
long integer
mybyte =
make8(num,3);
Extracts byte from 16- or
32-bit integer
MAKE WORD Make a 16-bit integer myword =
make16(byte1,
byte0);
Combine separate bytes into
one integer
MAKE LONG Make a 32-bit integer mylong = make32
(byte3,byte2,
byte1,byte0);
Combine 4 bytes or two
16-bit integers
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Table F.14: Type Conversions (#INCLUDE STDLIB.H)
Function Description Example Comment
ASCII TO
FLOAT
ASCII to float
conversion
num0 =
atof(decstring);
Converts a decimal
number as string into
float
ASCII TO
INTEGER
ASCII to 8-bit integer
conversion
num1 =
atoi(intstring1);
Converts an integer
given as string into an
8-bit integer
ASCII TO
LONG
ASCII to 16-bit integer
conversion
num2 =
atol(intstring2);
Converts an integer
given as string into a
6-bit integer
ASCII TO
32 BIT
ASCII to 32-bit integer
conversion
num3 =
atoi32(intstring3);
Converts an integer
given as string into a
32-bit integer
Table F.15: Character Test (#INCLUDE CTYPE.H)
Function Description Example Comment
ALPHANUMERIC? Test for alphanumeric
character
test =
isalnum(acode);
Returns 1 if character code
is in ranges 0–9, A–Z, a–z
NUMBER DIGIT? Test for numerical
digit character
test =
isdigit(acode);
Returns 1 if character code
is in range 0–9
LOWER CASE? Test for lower case
alphanumeric
test =
islower(acode);
Returns 1 if character code
is in range a–z
SPACE? Test for space
character
test =
isspace(acode);
Returns 1 if character code
is a space
UPPER CASE? Test for upper case
alphanumeric
test =
isupper(acode);
Returns 1 if character code
is in ranges A–Z
HEX DIGIT? Test for
hexadecimal digit
test =
isxdigit(acode);
Returns 1 if character code
is in ranges 0–9, A–F, a–f
CONTROL? Test for control
character
test =
iscntrl(acode);
Returns 1 if character code
is control code (00 – 1F)
GRAPHIC? Test for printable
character
test =
isgraph(acode);
Returns 1 if character code
is graphical (21 – 7E)
PRINTABLE? Test for printable
or space character
test =
isprint(acode);
Returns 1 if character code
is printable (20 – 7E)
PUNCTUATION? Test for punctuation
character
test =
ispunct(acode);
Returns 1 if character code
is a punctuation code
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Table F.16: Search and Sort (#INCLUDE STDLIB.H)
Function Description Example Comment
BINARY SEARCH Search for given
value in a data array
bsearch
(k,a1,n,w,compit)
Find value k in array
a1 of n elements of
width w
QUICK SORT Sort an array into
ascending order
qsort
(a1,n,w,sort1)
Sort array a1 of n
elements of width
w using function
sortit
Table F.17: Processor Controls
(Requires Chip Header File Only, e.g., 16F877A.H)
Function Description Example Comment
GET
ENVIRONMENT
Gets information
about the MCU
chip =
getenv(device);
Peripheral hardware,
memory, configuration,
etc.
GOTO ADDRESS Jump to program
memory location
goto_
address(0x1FF0);
Jump in ROM, use with
caution
LABEL ADDRESS Check address of
program label
labloc =
label_address
(start);
Labels should be used
only in exceptional cases
RESET CPU Restarts the
program from 0
reset_cpu(); No return
RESTART CAUSE Returns cause of last
reset
message =
restart_cause();
Messages defined in
MCU header file
RESTART
WATCHDOG
Clear watchdog
timer
restart_wdt(); Periodical operation to
prevent MCU watchdog
reset
SETUP
OSCILLATOR
Select internal clock
mode
setup_
oscillator();
MCUs with internal
clock
SLEEP Stops program and
waits for reset
sleep(); Wake up on specific
events
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Table F.18: Interrupts
(Requires Chip Header File, e.g., 16F877A.H #INT_XXXX)
Function Description Example Comment
INTERRUPT
DISABLE
Disables peripheral
interrupt
disable_interrupts
(int_timer0);
Interrupt labels
defined in device
header file
INTERRUPT
ENABLE
Enables peripheral
interrupt
enable_interrpts
(int_timer0);
Interrupt labels
defined in device
header file
INTERRUPT
CLEAR
Clears peripheral
interrupt
clear_interrupt
(int_timer0);
Interrupt labels
defined in device
header file
INTERRUPT
ACTIVE
Checks if interrupt flag
is set
interrupt_active
(int_timer0);
Interrupt labels
defined in device
header file
INTERRUPT
EDGE
Selects interrupt trigger
edge
ext_int_edge
(H_TO_L);
Rising (L_TO_H) or
falling (H_TO_L) edge
INTERRUPT
JUMP
Jump to address of ISR jump_to_isr
(isr_loc);
Use to service multiple
interrupts
Table F.19: Memory Read and Write
Function Description Example Comment
READ RAM
BANK
Read a RAM
location directly
abyte ⫽
read_bank(3,0x20);
Alternative variable
access
WRITE RAM
BANK
Write a byte into
user RAM
write_bank
(3,0x20,0xFF);
Write to bank 3,
address 0x20, data
0xFF
READ DATA
EEPROM
Read an EEPROM
location
abyte ⫽
read_eeprom(0x00);
Get byte at given
address
WRITE DATA
EEPROM
Write a byte into
EEPROM
write_eeprom
(0x1F,0x9A);
Write to nonvolatile
memory address, data
READ PROGRAM
ROM
Read code from
program ROM
read_program_memory
(0x100,copy,4);
Get block from
program address, copy
in RAM
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Table F.20: Memory Allocation (#INCLUDE STDLIBM.H)
Function Description Example Comment
MEMORY BLOCK
ALLOCATE
Reserves a block of
memory
ap1 =
calloc(25,4);
Allocated block = 25×5
bytes
MEMORY BLOCK
DEALLOCATE
Releases a memory block free(ap1); Previously allocated at
address pointer ap1
MEMORY BYTES
ALLOCATE
Reserves a number of
bytes
ap1 =
malloc(14);
Allocated block = 14
bytes
MEMORY BLOCK
COPY
Copy a given number of
bytes
memcpy
(ap1,ap2,n);
Copy n bytes from ap1
to ap2
MEMORY BLOCK
MOVE
Move a given number of
bytes
memmove
(ap1,ap2,n);
Move n bytes from ap1
to ap2
MEMORY BLOCK
SET
Initialize locations with a
given value
memset
(ap1,val1,
numofb);
Loads integer val1 into
numof locations from
ap1
Table F.21: Special Setup
(Requires Chip Header File Only, e.g., 16F877A.H)
Function Description Example Comment
SETUP WATCHDOG
TIMER
Initialize watchdog
time-out
setup_wdt
(wdt_1152ms);
Time-out options from
18 ms to 2.304 sec
RESET WATCHDOG
TIMER
Clear watchdog timer
within the program
loop
restart_wdt(); Watchdog timer is
normally reset before
time-out
SETUP
COMPARATORS
Connection of analog
comparators
setup_
comparator
(A0_A3_A1_A2);
Selected MCUs only
VOLTAGE
REFERENCE
Specify the comparator
ref. voltage
setup_vref
(vref_low|10);
Options in device
header file
SETUP OPAMP Enable built in op-amp
where fitted
setup_
opamp1(1);
Selected MCUs only
SETUP SLEEP Sets sleep delay time sleep_ulpwl
(time_in_us);
Selected MCUs only
LOW VOLTS
DETECT
Triggers interrupt if
supply low
setup_low_
volt_detect
(lvd_33);
Selected MCUs only
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Answers
Assessment 1
1. Musical birthday card, electronic price tag, sound system, television, automobile,
robot.
2. Input, ROM, CPU, RAM, output.
3. Flash ROM is non-volatile but reprogrammable, so the program can be changed
or the chip reused. Program testing and modification is easier and development
time is reduced compared with alternative types of program memory.
4. Number of I/O pins, program memory size, RAM size, EEPROM size, maximum
clock speed, range of interfaces, development system, cost, availability.
5. The program is stored as machine code instructions, executed in sequence. The
instruction register holds the current instruction and the program counter holds its
address. The file registers store the program data and the working register the data
being operated on.
6. 02 Program Counter Low Byte.
03 Status Register.
09 Port E Data Register.
89 Port E Data Direction Register.
20 General Purpose Register 1.
7. RC clock uses resistor/capacitor circuit to control clock frequency.
XT clock uses crystal circuit to control clock frequency.
WDT watchdog timer provides automatic reset if program hangs.
PUT power-up timer delays the program start until the MCU is ready.
NOWRT prevents writes to program memory areas.
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8. Tristate gate data switching circuit allows data through only when enabled;
otherwise, output is high impedance.
Current driver provides extra current on a loaded data line.
Data direction latch stores the bit that sets the port bit as input or output.
Input data latch stores the incoming bit when the port line is set to input.
Output data latch stores the outgoing bit when the port line is set to output.
9. 20-MHz clock → 5-MHz instruction clock → 200-ns period.
10ms 10,000,000ns.
Timer count required 10,000,000/200 50,000 instruction clock cycles.
Maximum count of 16-bit timer 65,536.
Preload value 65,536 50,000 15,536.
10. Resolution 2048/256 8mV per bit.
Output (1000/2048) 256 125 0 64 32 16 8 4 0 1
0111 11012.
11. The timer interrupt is set up at the beginning of the program. The timer is
started at some point in the program and runs concurrently with program
execution. When a time-out occurs, the program is suspended and the interrupt
service routine carried out. The program is then resumed at the original point.
Interrupts allow the timer to independently generate an accurate interval
between the timer start and interrupt request.
12. See the figure.
Idle S 0 0 0 0 0 P Idle 24V
1 1 1
13. RS232 is asynchronous, in that it has no separate clock signal. Instead, the
reception is resynchronized by each start bit, and reception is timed by a local
clock. SPI has a separate clock (used to strobe each bit into the receiver, generated
from the master MCU clock) and is therefore classed as a synchronous system.
14. SPI needs a hardware chip select signal connected to each slave, which the
master takes low to enable one slave receiver at a time. I2
C transmits the target
address on the data line; the slave must check all transmissions and pick up the
data that follow its own address.
15. I2
C has to send addressing and control information as well as the data on the
data line, while SPI has hardware slave selection.
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16. RS232 9600 baud 10 k bits/sec 1k bytes/sec 1000 characters/sec →
Page time 1 sec.
SPI 5-MHz clock → 0.2μs/bit → 2μs/character (some loading delay) →
Page time 2ms.
17. C source code entered via a text editor.
HEX hexadecimal code (machine code) program.
COF downloading file that contains the hex code plus debugging information.
LST list file, a text file containing source code, hex code, comments, etc.
ERR error file that lists the error messages generated by the compiler.
18. Vss 0V, Vdd 5V supply connections.
Vpp programming voltage (14V); !MCLR Master Clear resets MCU.
PGD program data download; PGC programming clock signal.
19. Project file shows the files used to make the project.
Source code edits window for entering program.
Disassembler list file shows the assembler code generated from the C source
code.
Output message shows the compiler status and errors.
Watch variable values monitored during program execution.
20. Host PC, MPLAB development system, C compiler, programming
module connectors, target system with PIC MCU.
Assessment 2
1. Include header file using #directive.
Main program statement block enclosed in braces.
I/O functions/sec within main.
2. Create MPLAB project.
Edit program using correct syntax.
Build program and correct syntax errors.
Test program in simulator and debug.
Optional—test in cosimulation mode.
3. output_C(64);
output_high(PIN_C6);
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4. The WHILE loop tests the control condition before the loop statements are
executed. The DO..WHILE tests after they have been executed at least once. The
FOR loop executes a loop a fixed number of times.
5. Port D bits initially go on for 1 sec. If the switch is active, the high 4 bits then
go off, and the program waits until the switch goes inactive, at which point all
the outputs go off. If the switch is inactive, all the LEDs go off after 1 sec.
6. (a) 255 (b) 32,767 (c) (2 – 1/223
) 2128
6.8 1038
7. (a) 8-bit precision 1/28
100% 0.39%.
(b) 32-bit FP precision 1/223
100% 0.000012%.
8. Mantissa 011 → 1
4 1
8 0.25 0.125 0.375 → 1.375.
Exponent 1000 0010 130 → 130 127 3 → 23
8.
Sign 0 → positive.
Number 8 1.375 11.000000.
9. a = n + 0x30;
putc(a);
10. n 5 01012, m 7 01112.
(a) 6, 0110 (b) 8, 1000 (c) 5, 0101 (d) 7, 0111 (e) 2, 0010
11. Continue means restart a loop, Break means quit a loop, Goto means jump to
a label unconditionally.
12. switch(x)
{ case 1: fun1();
break;
case 2: fun2();
break;
case 3: fun3();
break;
}
13. Local variables are allocated memory only when a function is called and are
discarded when the function has finished. The memory can then be used for
other purposes, saving on overall memory requirements.
14. Functions are self-contained blocks that implement a clearly defined set of
operations, receiving data for processing and returning results to the calling
routine. A structured program is a nested or hierarchical set of functions that
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is easy to understand and modify. Reusable function libraries can be created,
which save on programming time. Compiler packages provide function libraries
for the most common operations.
15. int variable type returned.
out name of the function.
int16 t variable and type received.
int16 n local variable declaration.
outbyte value returned from function.
16. The RS232 signal has a start bit, 8 data bits, and a stop bit. The edge of the start
bit triggers the LCD receiver shift register to sample the line in the middle of
each data bit. This is stored as an ASCII character and displayed. Control codes
for the LCD are preceded by the code 254.
17. See the figure.
Select Next Row
All done?
Key pressed?
Read Columns
Make key code
SCAN
RETURN
Yes
No
18. The function prints formatted output. This means that any variable output has
an associated formatting code, such as %d, which determines how the value
is interpreted. The main options are signed integer, floating point decimal, or
ASCII character. The variable anum in this case is an array variable, the element
being output is numbered n.
19. Ampersand () is the address_of operator, which causes the memory address of
the named variable to be returned. The pointer (*) is the contents of operator, which
returns the value of the contents of the location corresponding to the variable value.
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20. #include means copy another source code file into the user source code,
#define instructs the compiler to replace the given text with the given value,
#use means include a library function, #device defines the target MCU and
optionally an operating mode, #asm indicates the start of an assembly language
sequence,
Assessment 3
1. setup_adc_ports(AN0);
Reference 5V, resolution 5/1024 4.88mV/bit.
2. Resolution 4.096/1024 4.00mV/bit, conversion factor 0.004.
(a) float volts,input;
(b) volts=input*0.004;
3. enable_interrupts(int_AD);
enable_interrupts(global);
#int_AD
void isrADC(){}
4. Using the ADC interrupt, the program is more efficient because time is not
wasted in polling the ADC, and the ADC result can be processed as soon as it is
available.
5. 16-bit maximum count 65,536, remaining count 65,536 15,536 5,000.
Instruction clock 8/4 2MHz.
Clock period after prescale 16/2 8μs.
Timer period 5000 8 40ms.
6. The Capture mode uses an input bit change to trigger the capture of the current
timer reading, transferring it into the preload registers for processing. This mode
can be used for input signal period measurement. The Compare mode needs the
preload registers to be loaded with a value with which the current timer value is
continuously compared. An interrupt flag is set and an output toggled when they
match. This mode can be used to generate an output of a given period.
7. See the figure.
Duty Cycle d/1024%
Overall Period a*b*T
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8. Output period 1000μs 1000 clocks 250 4 timer count prescale →
setup_timer_2(4,250,1).
10% duty cycle 102/1023 → set_PWM1_duty(102)
.
9. The standard serial LCD is designed to receive 8-bit ASCII codes in RS232
format. High speed is not required, because only a limited amount of data is
sent as the display is updated. The longer-link distance possible with RS232
may be useful if the display is mounted away from the MCU board.
10. 0x41 is the ASCII code for character ‘A’. In the printf()statement, it is
output and displayed as a decimal 65 because the formatting code is %d. The
putc() function outputs the ASCII code and displays the character ‘A’.
11. The UART data transfer takes about 1ms, during which time the MCU could be
working on another task. MCU utilization can be increased by using interrupts,
which can be set up to fire when the serial port has finished sending (int_tbe)
or receiving (int_rda) a byte. The interrupt service routines contain the
code to write the next byte or read the next byte. On return from interrupt, a
foreground task continues, which is interrupted again only when the UART is
ready for the next byte transfer.
12. Each slave sender needs a slave select line connected to the master MCU, not
to ground. The master program contains bit switching statements to enable the
select line of a slave MCU programmed to transmit.
13. i2c_start();
i2c_write(0xA0);
i2c_write(0x01);
i2c_write(0xFF);
i2c_write(0xAA);
i2c_stop();
14. See the figure.
PIC1
SDA
5V
SCL
PIC2
SDA
SCL
15. Set up the PSP interrupt in the slave PIC.
Select the slave PIC by taking !CS low.
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Present the data to the Port D data pins.
Take !WR low to latch in the data.
Interrupt INT_PSP generated to read the port data.
16. The minimum number of wires is used by I2
C, but the rate of transfer is reduced
compared with SPI because control and address bytes have to be sent before the
data are returned.
17. (a) PSP (b) SPI (c) UART (d) I2
C
18. EEPROM is nonvolatile data storage, which allows data to be stored while the
power is off. It can therefore store security codes and limited amounts of other
key data long term. It is limited in size, so an external serial EEPROM can be
used to expand it.
19. The output speed is critical in this application, because the waveforms are
generated by outputting a table of values to the DAC as fast as possible. To
minimize the output loop time, interrupts are used instead of polling the
switches. The output frequency is thereby maximized.
20. An output bit can be toggled using an assembler sequence to minimize the
loop time, as shown in Section 2.8. In this circuit, the output port needs to be
switched between 0x00 and 0xFF using output_portD(n) within a minimal
loop to generate a fast square wave.
Assessment 4
1. See the figure.
PMOSFET
NMOSFET
Q1
0
0
1
1
Q2
Q3
Q4
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2. Speed 6 steps/sec 6 7.5 deg/sec 45 deg/sec 45/360 rev/sec →
60/8 7.5rpm.
3. Linear characteristic: Output voltage, Vt mt c; t temperature; Gradient,
m 10mV/°C.
At 0°C, sensor voltage, Vt 500mV, so 500 c. Hence, Vt 10t 500mV.
4. The parallel display uses more MCU output pins, drive requirements are more
complex (segment encoding required), and it shows only 3.5 numerical digits,
while the serial LCD is 16x2 alphanumeric.
5. lcd_symbol(DigMap[8],DIG1);
The first argument of the function is an array variable that contains the seven-
segment code for the number 8, and the second identifies the seven display
memory bits for the segments of the digit.
6. The DC motor needs position feedback to achieve a set position or speed. A
slotted wheel and optical sensor produce pulses as the shaft turns, allowing the
MCU to count the revs completed in unit time.
7. The stepper motor has multiple coils, which are energized in sequence to turn
the shaft, so it can be turned through a set number of steps with no feedback
required. The stepper motor on the mechatronics board has two sets of
windings, two wires each, which are connected to the four drive outputs.
8. Connect the motor sensor to Timer1 input and configure the timer to measure
the pulse period. The Capture mode of operation allows the timer count to be
captured when the sensor input changes. The MCU program can convert the
pulse period into revs/sec.
9. 1 step 7.5°, 1 rev 360/7.5 48 steps.
Time per step 1/48 20.8ms 21 ms.
10. The temperature sensor gives an output of 10mV/°C, with an offset of 500mV,
so the temperature can be calculated at any value in that range. The light sensor
output cannot be quantified in the same way, because it is not linear and the
absolute level is therefore more difficult to calculate.
11. Vt sensitivity temp offset (10 25) 500 750mV.
ADC output scaling 2048/1024 2mV/bit.
ADC output value Vt/scaling 750/2 375.
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12. Sink Pg.Ng M.N.F.
Source !Pg.!Ng = (P.M.!N.F)+(P.!M.F) = P.F.((M.!N)+!M).
13. P1 and N2, P2 and N1, M1 and M2. The current flows diagonally across the
bridge, so P1 and N2 are on together for forward current and P2 and N1 for
reverse. M1 switches on and off N2 and M2 switches N2 for PWM control.
14. With the inputs linked for full bridge operation, P1 and P2 operate Drives 1 and
2, respectively, which are connected to stepper motor Coil 1, brown and orange
wires. PWM1 is connected to CCP1 output. P3 and P4 operate Coil 2, red and
yellow; and PWM3 is connected to CCP2. Sequence: Drive 1, 4, 2, 3.
15. It is voltage operated with a high input impedance, so it is simple to interface
and can be driven directly from a logic output. The output ‘on’ resistance is low,
and the ‘off’ resistance is high.
16. Gain of amp 10.
Sensing resistor 0.1Ω.
Test resistor 3.3 0.5 3.8Ω.
Total resistance 3.8 0.1 3.9Ω.
Amp input voltage (0.1/3.9) 5 0.13V.
Amp output voltage 0.13 10 1.3V.
17. The latch consists of cross-coupled NOR gates, such that only one output can
be high at a time. The drives are disabled when the fault output is low and the
LED output is high. The comparator output goes high when an overcurrent is
detected, forcing the fault output low and switching on the LED. This state is
held until the Reset button forces the LED output low and the fault output high,
resetting the latch.
18. The MOSFET is switched by applying 5V between the gate and source, with
the load connected to the drain. The NMOSFET has its source connected to 0V
and is switched on with 5V at the gate; the PMOSFET has its source connected
to 5V and is switched on with 0V at its gate. This provides symmetrical drive
components in the half bridge.
19. Connect the motor between Drives 1 and 2. Enable drive at P1 from MCU RD7,
and control N2 from MCU CCP2(RD2). PWM output is generated from the
CCP2 module, which controls the speed of the motor.
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20. Output sequence at Port D: 0x80, 0x10, 0x40, 0x20.
PWM inputs not connected 1 (enabled).
Outputs high: RD7(P1 N2), RD4(P4 N3), RD6(P2 N1), RD5(P3 N4).
Drive sequence: Winding1 forward(Drive1 → Drive2).
Winding2 reverse(Drive4 → Drive3).
Winding1 reverse(Drive2 → Drive1).
Winding2 forward(Drive3 → Drive4).
Assessment 5
1. Hysteresis means that the switching level of the input depends on the polarity
of the input change. This helps overcome noise on the input, which would cause
unreliable switching, by implementing an upper and lower switching levels.
2. set_adc_channel(0);
numin read_adc();
In the read statement, the input value returned by the function has to be
assigned to another variable for processing. In the channel select statement, the
channel number is passed to the function as the function argument.
3. See the figure. (10 points)
Temperature
Controller
Sensor
Heater
Fan
Start Button
Stop Button
Temp OK Indicator
Running Indicator
4. TEMPCON
Initialize
MCU, ADC,Functions
Wait for 'Start'
Switch on 'Running'
Loop
Read temperature
If too low
Switch on Heater
If too high
Switch on Fan
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If OK
Switch on 'TempOK'
Always //
(10 points)
5. A data logger often needs to record analog input values from sensors. Flash
ROM is nonvolatile so data are retained during power off, and the serial
interface uses only two pins on the MCU. A serial link is needed to upload the
acquired data to a host system.
6. In a polled system, the time between input samples may vary if the processing
time changes between samples. A timer interrupt forces the execution of an ISR
containing the input sampling event at fixed intervals.
7. In a system with multiple interrupts, each is assigned a numerical priority in
relation to the others, such that a high-priority ISR is not interrupted by a lower-
priority one, but a low-priority interrupt may be interrupted by a high-priority
task.
8. The PC operating system is a priority-interrupt driven, multitasking OS
optimized for file processing, so that the time response of the system to real-
time events is not predictable. The real-time operating system is designed to
provide a predictable response time to major system events, as required in
control systems.
9. rate how often the task will execute.
max time allowed for this task each time it is executed.
10. The microcontroller has all essential hardware resources built into one chip:
CPU, program ROM, data RAM, and peripheral interfaces. In a conventional
microprocessor system, these are provided as separate chips so that the system
can be tailored to the application.
11. The system on a chip allows the microcontroller hardware to be configured for
a specific application then manufactured on one chip, giving the benefits of both
the conventional microprocessor system and the microcontroller.
12. Familiarity, cost, complexity, range, development system, availability, features.
13. Sufficient I/O pins, peripheral support, program memory size, data memory
size, speed, power consumption.
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14. The prototype costs are mainly hardware and software design time. As more
units are produced, the development costs are shared, so that the cost per unit
falls with the volume of production (see the figure).
Cost
per
Unit
Volume
15. The serial alphanumeric LCD needs only a single MCU pin and can display
several lines of numbers and characters. The 3.5-digit LCD is cheaper, the digit
display is larger, and access is faster.
16. The size of the system and number of components largely determine the
power consumption, plus the current drawn by the MCU increases with the
clock speed. The component data sheets need to be consulted to predict
power consumption, as this is not generally modeled in simulation systems. A
prototype must be built to confirm the power supply specification.
17. C is a higher-level language than assembler, so it is easier to learn and use, as
the meaning of the program statements is more obvious. The same standard C
syntax is used for all processors, with the compiler converting the source code
into the MCU-specific assembly language. This means that it is universal and,
to some extent, portable between systems. The basic programming techniques
are applicable to all embedded systems, with the main variation being in the I/O
function syntax. (10 points)
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Index
#bit 86
#byte 86
#define PIN_A0 40 86
#define T1_INTERNAL 0x85 86
#device ADC⫽8 directive 100
#fuses 11
#include 16F877A.h. statement 24, 39, 78
#int_ext directive 107
#list 86
#nolist 86
#pragma directive 230
#task directive 193
#use delay directive 66, 78, 212
#use rs232 78
#use rtos() directive 193
7-Bit ASCII codes 53
16F877 peripheral interrupts 105
16F877 primary interrupts 105
877 chip 9, 16
877 time registers 14
A
ACTIVE components 206
AD0–AD7 15
ALU (arithmetic and logic unit) 7
amp[n] array 131
Analog inputs 245
Analog setup 99–100, 101
Analog-to-digital converter (ADC) 12, 15
Animation options 224
ARES 203
Arithmetic and logical operations 54, 241
Arrays 74–5
Assembler block 88–92
Assignment operations 52–4, 55
Atmel (AVR) 196
B
BAR1 203, 204
list file 215–19
source code 211–12
BAR1.COF 221
BAR1.DSN 205, 221
BAR1.LST 215
BASE (basic application and system evaluation)
board 186–9
BCD count program 152, 153
Binary coded decimal (BCDx) digits 152
Blank program 47, 48
Block rotate 249
Break, continue, and goto 57–9
Breakpoint button 223
Brown-out Reset (BoR) 10
C
C compiler comparison 227
HiTech PIC C 227, 230–3
Matrix Multimedia C 227, 234–6
Microchip C18 227, 228–30
Mikroelektronika C 227, 234, 235
for PIC 16 series M 237
C interrupts 104–5
C mechatronics applications:
PICDEM 137
analog sensors 162–6
board simulation 171–5
DC motor test programs 152–6
liquid crystal display 146–52
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C mechatronics applications (Continued)
mechatronics board overview 137–46
stepper motor control 156–62
temperature controller 166–71
C peripheral interfaces:
PIC16 C 99
analog input 99–103
analog output 127–32
EEPROM interface 126–7, 128
hardware timers 108–13
I2
C serial bus 118–21
interrupts 104–8
parallel and serial interfaces 121–5
C program structure 60–2
C programming essentials:
PIC16 C 35–40
assembler routines 86–94
compiler directives 77–86
data operations 47–55
data types 73–7
functions and structure 60–4
input and output 64–73
program basics 40–7, 48
sequence control 56–60, 61
C Programming Language, The 23
C18 program 230
C18 User Guide 228, 229
CALC.C 69, 70–3
Capture, Compare, and PWM (CCP) modules 109
Capture hardware block diagram 112
Capture mode 111–13
CCP_1 112
CCS C:
16F877 header file 11
analog input functions 103
compiler 35
interrupt functions 104
program function reference 243
analog inputs 245
block rotate 249
character test 252
I2
C serial port 247
integers 251
interrupts 254
LCD control 248
math functions 249–50
memory allocation 255
memory read and write 254
parallel slave port 248
port input and output 244
processor controls 253
register manipulation 248
RS232 serial port 246
search and sort 253
special setup 255
SPI serial port 247
timers 245
trigonometric functions 251
type conversions 252
programming syntax 239
arithmetic and logic operators 241
compiler directives 239
formatting codes 241
I/O functions 240
program blocks 239
punctuation syntax 239
relational operators 240
variable types 240
software design:
BAR1 list file 215–19
BAR1 source code 211–12
PIC registers 212–15
source code components 48
CCS timer function 109
Character test 252
Character variable 52, 53
Clock options 9
Code protection (CP) 10
Communication links, comparison of 123–5
Comparator 15–16
Compare mode 111
Compiler directives 239
Component button 205
Conditional operations 54–5
Conventions, block diagram of 180
Counter/timer operation 108–9
CR mode 9
D
DC motor connections 142
Debugging 145–6
and testing:
application program 180–2
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Decision making 42, 43
delay_ms() 44
Digital controller, elements of 2
Digital input and output (I/O) 12–13
Digital to analog converter (DAC) 127
Disassembly Listing 145
DOWHILE.C 56, 57
DsPIC (digital signal processor) 8
Duty cycle 110
E
E-blocks 234
Electrically erasable programmable read only
memory (EEPROM) 11, 126
enable_interrupts(global); 108
enable_interrupts(int_ext);
statement 108
Enumeration 77
Erasable programmable ROM (EPROM) 1–2
ext_int_edge(H_TO_L); 108
External interrupt test hardware 107
F
FAST.C 89, 92
File select register (FSR) 214
Flash ROM 2
FLOAT.C 67–8, 69
Floating point (FP) 49–52
FOR loop 45
for statement 73
for(;;) 210
FORLOOP.C 45
Formatting codes 66, 68, 241
Frames per Second settings 224
Freescale (Motorola) 196
FUNC1.C 62, 63
function_name() 38
G
General purpose registers (GPRs) 7, 212
General timer operation 14
get_timer1() 156
getc() function 114
Global variable 63–4, 65
Globals option 223
H
Hardware design 179–80, 197–9
using ISIS schematic capture 203
design specification 203–4
schematic circuit 204–5
schematic edit 205–7
Hardware selection 195–6
Header file 78–86
Hitachi 196
HiTech PIC C 227, 230–3
Hold In Reset buttons 30
I
I/O allocation, for temperature controller 168
I/O functions 240
I/O pin operation 13
I2
C serial port 247
i2c_start() function 120
If..else and switch..case 59–60, 61
if statement 42, 163, 226, 219
IFIN.C 42, 43
In-circuit programming and debugging (ICPD)
10–11
In-circuit serial programming (ICSP) 27
INBIT.DSN 42, 43
include directive 212
include files 224
include statement 36, 212
Indirect addressing operators 75–7
Input voltage measurement and display 102
input(PIN_nn) 44
Instruction set, for programming 90–1, 93–4
INTCONbits.TMR0IF⫽0 statement 230
Integers 49, 251
Intel 8051 2
Interintegrated circuit (I2
C) serial bus 21–2, 118
functions 121
test system 120
Interrupt 254
operation 190–1
in PIC16 peripherals 16–18
statements 107–8
Interrupt service routine (ISR) 17, 190
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Inversion error 224
ISIS schematic capture:
hardware design 203
design specification 203–4
schematic circuit 204–5
schematic edit 205–7
K
Keypad and calculator 68–73
L
Labcenter 203, 227
lcd.inc 152
lcd_symbol() function 151, 152, 156
LCDDATAx 147
Least significant bit (LSB) 108
LED flasher program 229
Light sensor 162–3, 164
Linker errors 224
Liquid crystal display (LCD) 146
connections 146–9
control 248
segment connections 147
test program 149–52
Local variable 63–4, 65
Logical errors 224
Loop control 42, 44
Looping 41–2
Low-cost in-circuit debugging 2
Low pin count (LPC) 8
Low-voltage programming mode 11
LP (low-power) mode 9
M
main() function 60, 62, 73, 210
makenum() 73
Mantissa 49
math.h library 131
Math functions 249–50
Matrix Multimedia C 227, 234–6
Mechatronics board fixed connections 141
Mechatronics board user connections 142
Memory allocation 255
Memory read and write 254
Microchip 227
Microchip C18 227, 228–30
Microchip MPLAB ICD2®
29
Microchip PICkit2 programmer 27
Microchip® Inc. 137
Microcontroller unit (MCU) 1
configuration 8–12
features 3, 4, 5
programs 35
Microcontrollers 196–7, 198
types 8
Microsoft Windows®
191
Mikroelektronika C 227, 234, 235
Motor drives 138, 139, 142, 143
MPLAB 2, 10–11
C project 23–4
ICD2 145
IDE screenshot 39
MPSIM 181–2
N
National Semiconductor 196
“Null terminated string” 75
O
Object Selector panel 205
Operating System (OS) 191
Oscillator start-up timer 10
out() function 62, 64
outbyte.c 25
outbyte.cof 25
outbyte.err 26
outbyte.hex 25
outbyte.lst 25
outbyte.mcp 26
outbyte.mcw 26
outbyte.pjt 26
outbyte.sym 26
OUTBYTE test circuit 38
OUTNUM.COF 37
output_D(255) 38
output_high(PIN_nn) 44
Overview window 206
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P
Parallel slave port (PSP) 121–3, 248
functions 124
operation 16
Parameter error 225
PC operating system 191–2
PCLATH (program counter latch high)
215
Permanent magnet (PM) 157
Philips 196
PIC.H 232
PIC 16F877 65
file registers 213, 214
status register bit functions 215
PIC 16FXXX instruction set 90–1, 93–4
PIC assembly language 92–4
PIC chips 7–8
PIC registers 212–15
PIC16:
MCU configuration 8–12
clock options 9
configuration, in C 11–12
options 9–11
microcontrollers 1–8
MCU features 3, 4, 5
PIC chips 7–8
program execution 3–5
RAM file registers 6, 7
MPLAB projects 22–6
MPLAB C project 23–4
project files 25–6
peripherals 12–18
analog-to-digital converter 15
comparator 15–16
digital I/O 12–13
interrupts 16–18
parallel slave port operation 16
timers 13–15
program and debug 26–30
debugging 29–30
design package 30
programming 27–8
serial interfaces 18–22
interintegrated circuit bus 21–2
SPI bus 20, 21
USART 19–20
PIC16 C 35
analog input 99
analog setup 99–100, 101
voltage measurement 100–3
analog output 127–32
application design 179
debugging and testing 180–2
hardware design 179–80
software design 180, 181
applications and systems 179
data logger system 184–9
design 179–82
operating systems 189–94
system design 195–200
temperature controller 182–4,
185
assembler routines 86
assembler block 88–92
PIC assembly language 92–4
program compilation 87–8
compiler directives 77
header file 78–86
program directives 78
data logger system 184
BASE board 186–9
program outline 188, 189
data operations 47
assignment operations 52–4, 55
conditional operations 54–5
variable types 47–52
data types 73
arrays 74–5
enumeration 77
indirect addressing operators 75–7
EEPROM interface 126–7, 128
functions and structure 60
arguments 62–3
global and local variables 63–4, 65
program structure 60–2
hardware timers 108
capture mode 111–13
compare mode 111
counter/timer operation 108–9
PWM mode 109–11
I2
C serial bus 118–21
input and output 64
keypad and calculator 68–73
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PIC16 C (Continued)
RS232 serial data 64, 65
serial LCD 65–8, 69
interrupts 104
C interrupts 104–5
example 106–7
interrupt statements 107–8
operating systems 189
interrupts 190–1
PC operating system 191–2
polled I/O 189–90
RTOS 192–4
parallel and serial interfaces 121
communication links, comparison of 123–5
parallel slave port 121–3, 124
program analysis 38–40
program basics 40
blank program 47, 48
decision making 42, 43
FOR loop 45
loop control 42, 44
looping 41–2
SIREN program 45–7
variables 40–1
program creation 36
program testing 37–8, 39
sequence control 56
break, continue, and goto 57–9
if..else and switch..case 59–60, 61
while loops 56–7
serial bus 116–18, 119
simple program 35–6
system design 195
hardware design 197–9
hardware selection 195–6
microcontrollers 196–7, 198
software design 199–200
temperature controller 182
software design and implementation 183–4,
185
system operation 182–3
UART serial link 113–15
PIC16F84 2
PIC16F877A 3
PICCLite 232
PICDEM:
analog sensors 162
light sensor 162–3, 164
temperature measurement 163–6
board simulation 171, 172
circuit description 173–5
DC motor test programs 152, 154
control 152, 154
Rev counter 154–6
hardware 137–8, 139, 140, 141, 142, 143
liquid crystal display 146
BCD count program 152, 153
LCD connections 146–9
LCD test program 149–52
mechatronics board overview 137, 138
debugging 145–6
motor drives 138, 139, 142, 143
PICDEM hardware 137–8, 139, 140, 141,
142, 143
test program 143–5
stepper motor control 156
construction 157–8
direction control 158–62
stepper motor test 158, 159, 160
temperature controller 166
I/O allocation 168
implementation 168–71
specification 166–7
Pick Devices button 205
PICkit2.exe file 28
POINTS.C 76
Polled I/O 189–90
Power-up timer (PuT) 10
printf() 75, 114
Processor controls 253
Program analysis 38–40
Program blocks 239
Program compilation 87–8
Program Configuration Word 221
Program creation 36
Program debugging 221–4
Program directives 78
Program execution 3–5
Program simulation screenshot 223
Program Target Device 145
Program testing 37–8, 39
Programming, instruction set for 90–1, 93–4
Project files 25–6
PROJNAME.C 23, 24
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PROJNAME.HEX 23
Proteus VSM 182, 203, 205
system testing:
errors 224–6
program, attaching 221
program, debugging 221–4
Pulse Width Modulation (PWM) mode 14, 109–11
Punctuation syntax 239
putc(13) function 114
putc(code) 66
R
RAM file registers 6, 7
read_ADC() function 100
Real-time application flowchart 209
Real-time counter clock (RTCC), see Timer0
Real-time operating system (RTOS) 18, 192–4
Register 0C (1210) 93
Register manipulation 248
Relational operators 240
Release From Reset buttons 30
Rev counter 154–6
RISC (reduced instruction set computer)
processor 4
ROM (read only memory) 1, 2
RS232 20
peripheral simulation 115
serial data 64, 65
serial port 246
functions 114
rtos_disable(task1) 194
rtos_enable(task1) 194
rtos_run() statement 193
rtos_yield() function 194
S
Salvo RTOS 232
Salvo RTOS User Manual 194
Search and sort 253
Sequence error 224
Serial LCD 65–8, 69
Serial peripheral interface (SPI) bus 20, 21
function set 116
serial port 247
test system 117
set_ADC_channel(n) function 99
Set Font option 223
set_pwm1_duty() function 110
set_up_adc_ports() system function 99
setup_adc() function 102
setup_adc_ports() 102
setup_ccp1() function 110
setup_lcd() function 151
setup_timer_1 156
setup_timer_2() function 110
setwave() function 129
Signed integers 49
Simple program 35–6
Single analog input and display test circuit 100
SIREN program 45–7
SIREN.C 45
Software design 180, 181, 199–200
and implementation 183–4, 185
using CCS C 209
BAR1 list file 215–19
BAR1 source code 211–12
PIC registers 212–15
Special function registers (SFRs) 7, 212
Special setup 255
SPICE model 206
Step Out Of 222
Step Over 222
STEPDIR.C 158
Stepper motor connections 143
Stepper motor test 158, 159, 160
STEPSPEED 158
STEPTEST.C 158
STMicroelectronics 196
strcpy() function 75
String 74
Syntax error 224
System operation 182–3
System testing, using Proteus VSM 221
errors 224–6
program, attaching 221, 222
program, debugging 221–4
T
T0CKI (Timer0 clock input) 109
Tagged Object edit buttons 207
Temperature control system 183
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Temperature measurement 163–6
Terminal button 206
Test program 143–5
Test program debugging screen 146
Timer interrupt process 18
Timer0 14, 109
Timer1 14–15, 109, 190–1
Timer2 15, 190
Timers 13–15, 245
Timestep per Frame settings 224
Timing error 225–6
Trigonometric functions 251
Type conversions 252
U
Universal synchronous/asynchronous receive
transmit (USART) 19–20
V
Variables 40–1
types 47, 240
character variable 52, 53
floating point 49–51
integers 49
signed integers 49
Virtual oscilloscope screenshot 225
void change() 132
void isrext() 107
void main() 36, 38
Voltage measurement 100–3
W
while loops 41, 45, 56–7
while(1) statement 73, 210, 219
WHILOOP.C 44
Watchdog timer (WDT) 9–10
Waveform generator 129
X
XT mode 9
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